It might the possible that the PDF files are corrupted.
Please transfer PDF files on your computer & open it using Reader DC application, check if you are able to view the text.
This sounds very much like the new bug that we have identified in Acrobat Reader for iOS version 15.3.0.
The good news is that we already have a fix for it. We are trying to release a new update to the App Store soon.
Would you be willing to share your PDF document with us (Adobe)? We can confirm if the defect has been fixed correctly using our internal build and your PDF document.
I sent you a separate private forum message with the instructions on how to share a PDF document with us.
Thank you for your help!
We just released Acrobat Reader for iOS version 15.4 on January 23, 2016.
This release includes the fix for the particular defect that you have encountered.
Would you please give it a try and let us know if it works for you?
If you have trouble updating Acrobat Reader from the Apple App Store, please take a look at the following page for tips: http://adobe.com/go/updatehelp