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I am using Adobe Acrobat Pro DC version 2021.005.20060. If I create a PDF and add multiple PDFs, some of those files are corrupted after combining them with the others. The text on those files is turned into boxes and other shapes and totally illegible. I have been able to narrow it down to a few files that consistently corrupt when trying to combine them, so there is something unique about them that Acrobat does not like. Any thoughts on how to resolve? If not, I would like to submit these files to you so that you can recreate the problem and advise on how to resolve it. Thank you.
Check the fonts on the individual PDFs to confirm they are embedded- File> Properties> Fonts. All should say embedded or embedded subset.
If some fonts are not embedded, the best option is to request a new PDF with embedded fonts, if that's not possible, try running a Preflight fixup to embed the fonts, then try to combine again. Tools> Print Production> Preflight> Fixups.
The solution does not appear to be font related but rather related to how Acrobat saves a new pdf after combining multiple pdfs together using its "Combine Files" tool. That is, if I click "Reduce File Size" under File Options, the new pdf appears to be saved without any page corruption. However, if I save the same exact pdf without checking the "Reduce File Size" option, the same pages are consistently corrupted. There is no difference in the fonts used on the pages corrupted and those not corrupted. This bug also only seems to appear after combining multiple pdf's into one using Acrobat's Combine File tool. So, while I have a workaround by always reducing the file size when saving, this seems to be a consistent and repeatable bug that will hopefully be fixed in a new release of Acrobat.
This may indeed be a bug, you can report it here:
I suspect other people would have noticed this issue and it would have been reported and addressed.
When you combine PDFs, I believe Acrobat attempts to consolidate the fonts, sometimes it does not do it correctly, particularly if some of the fonts are not embedded. Using Reduced file size may be optimizing the fonts, which appears to fix your issue, however it may also be reducing your image resolution and converting to RGB. Does your combined PDFs have embedded fonts? If not, you may see unexpected results when you print it. As a test, you could also try saving your original PDFs as PDF/X-1a before combining, and/or saving them as optimized PDFs (File> Save-as other> Optimized PDF.