http://lilithlela.cyberguerrilla.org/?p=603

This blog intends to augment USP no. 3: The danger of metadata [digital footprints] (original here) for *nix distros:

In order to modify the dates and times you’ll need to use a hex editor to manually change the data yourself. A simple free hex editor for Windows is called HexEdit (http://www.expertcomsoft.com/download.htm) and will allow you to perform the changes you need to the PDF file that PDF Info and Acrobat don’t allow you to (you can also always open the PDF file in Notepad, but this can take a while and will cause slower computers hang).

Note: The link http://www.expertcomsoft.com/download.htm is no longer leading to anything useful, but that is not a problem. There are many hexeditors available for windows like Hack, Hexplorer and Frhed.

An alternative to using hexedit is using sed, see here (at bottom: the less elegant scrubbing)

For *nix

hexedit is a program that gives the user the ability to view a file in hexadecimal and ASCII view. It offers the ability to read a device as a file. It includes build in key shortcuts to make it fast and easy to edit and analyze file, including skipping to specific memory locations, cutting and pasting, changing views, modes, and syntaxes similar to that of emacs: online manpage on die.net

Note: You can also use vi as a hex editor. It isn't a real 'hex mode', what actually happens is that vi’s buffer is streamed through the external program ‘xxd’, but it works well for some cases of scrubbing.

Open a file in vi as usual, hit escape and to switch into hex mode type:
:%!xxd

And when your done, to exit from hex mode, hit escape again and type:
:%!xxd -r

For MacOS

HexEdit is a Macintosh hexadecimal file editor that handles both data and resource forks. It is the most widely used editor of it's kind on the platform, and can handle file sizes of up to four gigabytes with ease, yet has memory requirements of less than a megabyte (with color turned off). It can compare whole files for similarities or differences and will soon be able to do so for any subset within the files. You can download it from sourceforge.

Usage for scrubbing pdf's

Be sure to backup your data before operating any hexedit code.

In general, switch to ASCII mode, turn off "read only" mode, and start searching through the file. For scrubbing pdf's look for "created" (metadata appears in the PDF file more than once). If and when you find metadata, change to fake data or delete. Then repeat your search again for the terms ‘create,’ ‘creation,’ ‘modified,’ and ‘modify,’ and similarly either replace or delete the dates, once again being sure to repeat each search so that any potential multiple instances of the field can be located and modified or blanked out.