Author Topic: Can the recipient modify a PDF?  (Read 1028 times)

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Can the recipient modify a PDF?
« on: December 03, 2013, 04:42:21 PM »
If I create a PDF file of scanned pages, attach it to an email and send it (of course,) can the recipient modify that particular file? Can I modify the original PDF here, on my computer?
 --- Paul

With the proper skills and software, anything can be modified; although in a complex file format the corruption will leave more-or-less visible evidence if not of the actual tampering, of the fact of a likely modification. (I can probably change a basic ASCII text file without leaving tracks.)

If you create the PDF from an electronic document such as by printing from a word processor to a PDF driver or using a basic program, the characters of the text are stored as text. You can prove this by selecting and copying small sections (usually no more than one line at a time) of text from within the viewer and pasting it into another document. These PDFs could be opened in a commercial program such as those from Adobe, Nuance, Foxit and others and edited more or less as if it were any other object-based document.

When a scanner saves its output as a PDF, it is generally wrapping an image – a TIFF or JPG – in a PDF header that tells the computer what interpreter to use to render the image. A skilled user could extract the image and edit it with the same process she would use to remove an ex from a family photograph but could not extract or modify individual characters of text. Occasionally a higher end scanner with a significant software suite will attempt to OCR your scan. Usually these programs don’t result in a PDF, but rather dump it into a word processor or spreadsheet as a fully editable document.

The commercial PDF software generally has a feature in their higher-priced editions to password protect and lock their output to permit only viewing or only viewing and printing. Privacy software such as PGP can create a digital fingerprint and signature that will reveal the change of as little as a single byte of a file. Open-source publishers will often offer you the very small “MD5” or “SHA-1” hash or other separate file to verify that their program has not been corrupted by a malicious hacker before distributing it. Unfortunately use of these signatures is limited to only the most tech-savvy users. Even Glenn Greenwald, the investigative reporter contacted by Edward Snowden, was unable to use basic procedures to protect his sources.

Bill Barnes