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I have one question regarding the pdf structure, why % sign is used in pdf.

I got some results where they mentioned % sign is used for comments but if we use % sign for comments then what about % sign used in %PDF-1.5 and %%EOF?

%PDF-1.5 which defines the header of the file and

%%EOF which defines the end of the pdf structure.

then why is the % sign used for PDF-1.5 and why is the % sign used 2 times in EOF?

From the results I knew that % sign is used for comments, so why it is different for above two terms?

Your help will be appreciated - Thank you

Aimee Borda
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Unmesh Suryawanshi
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    Why exactly have you tagged your question with [tag:malware] and [tag:security]? – mkl Mar 16 '17 at 10:01

2 Answers2

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I actually know nothing about pdf structure or using % correctly, but it seems to have the same reasons as the shebang #! followed by an executable is required in shell scripts, like bash, perl, or even python.

More can be read at this stack overflow answer here for why bash scripts need the #! at the beginning of scripts: https://stackoverflow.com/a/8968514/6037755

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Graytr
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why is the % sign used for PDF-1.5 and why is the % sign used 2 times in EOF?

From the results I knew that % sign is used for comments, so why it is different for above two terms?

You can consider those entries actually to be comments (after all they do not contain any PDF objects as such to use for PDF rendering) which you are required to put at certain positions of a PDF file.

According to the specification ISO 32000-1:

7.5.2 File Header

The first line of a PDF file shall be a header consisting of the 5 characters %PDF– followed by a version number of the form 1.N, where N is a digit between 0 and 7.

and

7.5.5 File Trailer

The trailer of a PDF file enables a conforming reader to quickly find the cross-reference table and certain special objects. Conforming readers should read a PDF file from its end. The last line of the file shall contain only the end-of-file marker, %%EOF.

As the use of "shall" here indicates, these are requirements.

And it indeed makes sense that these markers in all other respects are comments.

Only for the purpose of identifying the start and the end of a PDF these markers have a special meaning, before a PDF processor starts working with actual PDF objects. As soon as start and end are identified, these markers have to be ignored. So, making these markers comments is an obvious choice.

This is true for unusual processing types, too. E.g. if for some reason the cross references of a PDF are broken and some program tries to re-create them by searching for indirect PDF objects, it does not need to specially treat these markers, it automatically ignores them as comments.

PS According to Adobe's Implementation Notes in the Annex H of their PDF Reference, their tools also accept an alternative header:

3.4.1, “File Header”

[...]

14.Acrobat viewers also accept a header of the form

%!PS−Adobe−N.n PDF−M.m

If you want to find out why the marker comment contents were chosen exactly like they they are, therefore, you should look into the history of PDF and Postscript

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mkl
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