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#1 Posted : Tuesday, March 17, 2020 2:12:19 AM(UTC)

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Hi Paul,

Please see a previous issue we had and raised with you - https://forum.patagames....Acrobat-reader#post1348. It seems your suggestion in there, to save with the Increment flag, works. When I do this I am able to open the resulting file in Adobe Acrobat Reader DC without the warning and edit it.

My problem is that our code has always used the NoIncrement flag when saving, so all the PDFs in our software have been saved in that form. I've tried re-saving one of these PDFs incrementally, but that doesn't help. As far as I understand it, this is something to do with the signature on the original file. Saving incrementally seems to preserve this signature, meaning the PDF can still be opened properly in Acrobat Reader. But saving with the NoIncrement flag seems to re-write the entire file (and corrupt the signature). So I'm guessing there's no way back for these files.

This is probably more of a general PDF question than one specific to Patagames, but am I on the right track? Is there no way to "revert" a PDF that's been saved with the NoIncrement flag?


Paul Rayman  
#2 Posted : Wednesday, March 18, 2020 12:04:36 AM(UTC)
Paul Rayman

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No, revert is not possible. NoIncremental save completely reconstructs the document. While Incremental save just appends modifications to the document. And once a document has been signed, all changes made to the document must be saved using incremental updates, since altering any existing bytes in the file invalidates existing signatures.

Moreover, applications may allow users to modify PDF documents. Users should not have to wait for the entire file—which can contain hundreds of pages or more—to be rewritten each time modifications to the document are saved. PDF allows modifications to be appended to a file, leaving the original data intact. The addendum appended when a file is incrementally saved contains only those objects that
were actually added or modified, and includes an update to the cross-reference table. Incremental save allows an application to save modifications to a PDF document in an amount of time proportional to the size of the modification rather than the size of the file.
In addition, because the original contents of the document are still present in the file, it is possible to undo saved changes by deleting one or more addenda. The ability to recover the exact contents of an original document is critical when digital signatures have been applied and subsequently need to be verified.
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