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Update pdfio.md
addition of lines requeested
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doc/pdfio.md
@ -135,15 +135,14 @@ PDFio exposes several types:
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Understanding PDF Files
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-----------------------
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A PDF file is structure in a way, so that it would be displayed in the same way
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across multiple devices and platforms. The basic structure of PDF File is as follows:
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### A small PDF File
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The following is a PDF which says "Hello, World" on one page:
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A PDF file provides data and commands for displaying pages of graphics and text,
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and is structured in a way that allows it to be displayed in the same way across
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multiple devices and platforms.
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The following is a PDF which shows "Hello, World!" on one page:
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```
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%PDF-1.0 **Header starts here**
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%PDF-1.0 %Header starts here
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%âãÏÓ
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1 0 obj **Body starts here**
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1 0 obj %Body starts here
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<<
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/Kids [2 0 R]
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/Count 1
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@ -190,7 +189,7 @@ endobj
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/Type /Catalog
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>>
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endobj
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xref **Cross-reference table starts here**
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xref %Cross-reference table starts here
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0 6
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0000000000 65535 f
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0000000015 00000 n
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@ -198,7 +197,7 @@ xref **Cross-reference table starts here**
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0000000192 00000 n
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0000000291 00000 n
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0000000409 00000 n
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trailer **Trailer starts here**
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trailer %Trailer starts here
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<<
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/Root 5 0 R
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/Size 6
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@ -210,7 +209,7 @@ startxref
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### Header
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This is the first line of a PDF File. This specifies the version of PDF Format used.
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- Example: '%PDF-1.0'
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For Example: '%PDF-1.0'
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Since PDF files almost always contain binary data, they can become corrupted if line
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endings are changed (for example, if the file is transferred over FTP in text mode).
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@ -228,8 +227,7 @@ character codes in excess of 127. So, the whole header in our example is:
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### Body
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The file body consists of a sequence of objects, each preceded by an object number,
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generation number, and the obj keyword on one line, and followed by the endobj keyword
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on another.
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- For Example:
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on another. For Example:
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```
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1 0 obj
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@ -252,40 +250,40 @@ Objects that are not used are never read, making the process efficient.
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Operations like counting the number of pages in a PDF document are fast, even in large files.
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Each object has an object number and a generation number.
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- Generation numbers are used when a cross-reference table entry is reused.
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- For simplicity, we would assume generation numbers to be always zero and ignore them.
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- For simplicity, we will assume generation numbers to be always zero and ignore them.
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The cross-reference table consists of:
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- Header line that indicates the number of entries.
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- Special entry (the first entry).
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- One line for each of the object in the file body.
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```
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0 6 **Six entries in table, starting at 0**
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0000000000 65535 **f Special entry**
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0000000015 00000 **n Object 1 is at byte offset 15**
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0000000074 00000 **n Object 2 is at byte offset 74**
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0000000192 00000 **n etc...**
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0000000291 00000 **n**
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0000000409 00000 **n Object 5 is at byte offset 409**
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0 6 %Six entries in table, starting at 0
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0000000000 65535 f %Special entry
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0000000015 00000 n %Object 1 is at byte offset 15
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0000000074 00000 n %Object 2 is at byte offset 74
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0000000192 00000 n %etc...
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0000000291 00000 n
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0000000409 00000 n %Object 5 is at byte offset 409
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```
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### Trailer
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The first line of the trailer is just the trailer keyword. This is followed by the trailer dictionary,
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which contains at least the /Size entry (which gives the number of entries in the cross-reference table)
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and the /Root entry (which gives the object number of the document catalog, which is the root element
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which contains at least the /Size entry (Number of entries in the cross-reference table)
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and the /Root entry (Object number of the document catalog, which is the root element
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of the graph of objects in the body).
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There follows a line with just the startxref keyword, a line with a single number (the byte offset of
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the start of the cross-reference table within the file), and then the line %%EOF, which signals the
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end of the PDF file.
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```
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trailer **Trailer keyword**
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<< **The trailer dictinonary**
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trailer %Trailer keyword
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<< %The trailer dictinonary
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/Root 5 0 R
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/Size 6
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>>
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startxref **startxref keyword**
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459 **Byte offset of cross-reference table**
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%%EOF **End-of-file marker**
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startxref %startxref keyword
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459 %Byte offset of cross-reference table
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%%EOF %End-of-file marker
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```
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How a PDF File is Read
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@ -302,7 +300,8 @@ table retrieved.
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6. At this stage, all the objects can be read and parsed, or we can leave this process until each
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object is actually needed, reading it on demand.
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8. We can now use the data, extracting the pages, parsing graphical content, extracting metadata,
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and so on. This is not an exhaustive description, since there are many possible complications
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and so on.
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This is not an exhaustive description, since there are many possible complications
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(encryption, linearization, objects, and cross reference streams).
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How a PDF File is Written
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