There are a number of ways you can do what you describe, including hidden form fields, pages, document icons, document metadata, etc. The problem is it cannot be considered secure and is relatively easy to defeat.
Some more questions: What type of serialization are we talking here? Should
it be visible or hidden? Can the file be encrypted after the serialization
is implemented? How secure should it be?
I would prefer a combination of both visible and hidden serialization; a prominent watermark across the page (in the event someone was to print a page of the manual to disclose to unauthorized persons), as well as an unseen field that could be viewed in properties on a digital copy.
The main thing is that we have 250+ employees, so generating a digital copy of the manual for each employee would have whatever number assigned to the respective recipient that we could distribute, and then later use this number for tracking any source of the leaked manual. The manual has response procedures to critical incidents and some other topics that need not be available to the public. My suggestion was to remove the sensitive sections altogether... but that is not to be.
This really seems like something best suited for certificate-based security, but I know my agency will not entertain the thought of paying for that. If I can just get a way to generate the 250+ copies in a batch process, that would be great.
Have a look at this tool, which I have just recently published:
It can be used to create multiple watermarked versions of the same file
based on text from a list.
As you can see from the screenshots, you can set the font properties,
location of the text and also its transparency level. So you can use it to
add visible or hidden watermarks.
After adding the watermarks you can run a batch process to secure the files,
thereby making it even more difficult to mess around with them.
You can also send me an email to try6767 at gmail dot com to discuss it