I am using Adobe Acrobat Pro DC version 2018.011.20058, but have had the same problem with Adobe Acrobat X version 10.1.1.6. 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. 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.