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@ -286,40 +286,6 @@ startxref %startxref keyword
%%EOF %End-of-file marker %%EOF %End-of-file marker
``` ```
How a PDF File is Read
----------------------
To read a PDF file, converting it from a flat series of bytes into a graph of objects in memory,
the following steps might typically occur:
1. Read the PDF header from the beginning of the file, checking that this is, indeed, a PDF
document and retrieving its version number.
3. The end-of-file marker is now found, by searching backward from the end of the file.
The trailer dictionary can now be read, and the byte offset of the start of the cross-reference
table retrieved.
5. The cross-reference table can now be read. We now know where each object in the file is.
6. At this stage, all the objects can be read and parsed, or we can leave this process until each
object is actually needed, reading it on demand.
8. We can now use the data, extracting the pages, parsing graphical content, extracting metadata,
and so on.
This is not an exhaustive description, since there are many possible complications
(encryption, linearization, objects, and cross reference streams).
How a PDF File is Written
-------------------------
Writing a PDF document to a series of bytes in a file is much simpler than
reading it—we don’t need to support all of the PDF format, just the subset
we intend to use. Writing a PDF file is very fast, since it amounts to little
more than flattening the object graph to a series of bytes.
1. Output the header.
2. Remove any objects which are not referenced by any other object in the
PDF. This avoids writing objects which are no longer needed.
3. Renumber the objects so they run from 1 to n where n is the number of
objects in the file.
4. Output the objects one by one, starting with object number one,
recording the byte offset of each for the cross-reference table.
5. Write the cross-reference table.
6. Write the trailer, trailer dictionary, and end-of-file marker.
Reading PDF Files Reading PDF Files
----------------- -----------------