My problem is that in scientific documents / books (most PDF) You have the last chapter something like this:
Recommended further readings
Checking ISBN in pdf - K.I. Wi Dude. ISBN:1234567891234
EPub and PDF Pro's and cons. - dr. Merry ISBN: 4321987654321
About the author
Robin Hood is a financial expert.
Calibre Publishing 2011
So as you can see, the first 2 ISBN-numbers are not related to the current document. The last one is.
This is often seen in scientific documents.
The real ISBN is on one of the first pages,
After the further reading section
or not in the document at all.