I have several PDF files created from Word documents.
Here's where it gets crazy:
When I try to merge them, some of the PDF files revert to a previous state of the Word document.
Errors I had previously corrected in Word reappear in the PDF file!
For example: The sentence "There are 22 units." was changed to "There are 16 units." in the Word document. After printing as PDF, it still said 16. After combining this with other documents, it still says 16. After saving the "Binder1.pdf" file and opening it again, it suddenly says "There are 22 units." again.
This is really bad, I luckily caught this by accident, but it would have been a nightmare if this had gone out to the client.
The word files were created cooperatively with Microsoft Word on a Sharepoint and then printed as a PDF using Adobe Acrobat's PDF printer. This is on a tightly secured corporate computer, no third-party software.
"Save as PDF" doesn't help either. Saving as PDF/A doesn't help. The only way around this is to use the "Append pages" functionality instead of "Combine PDF files".
Just found my mistake. Added the same document twice, kept scrolling down to the first (old) document. I'm an idiot.