Since the most recent DTTG update, I’ve been experiencing a problem where some PDFs I’ve downloaded from an academic database (HeinOnline) are not viewable within DTTG. I’m able to view the same PDFs on DEVONthink on my Mac, and am able to view them in PDF Expert if I drag the file (on my iPad) from DTTG into PDF Expert.
I’ve previously been able to view (and have annotated) these same PDFs in DTTG, and the error doesn’t affect all of my PDFs – it seems to be affecting only PDFs of older documents that HeinOnline added by scanning, rather than more recent documents for which PDFs were generated directly from the electronic source. Even as to older documents, the problem doesn’t exist with all documents, and for the documents where it applies, I’m still able to see the pages of the PDFs, but they appear blank (with highlighting showing on pages where I’d previously made annotations).
I’m curious whether others are experiencing the same problem, and whether there are any suggested fixes.
It’s impossible to say without looking at your particular PDFs. All PDFs are not created equal. 
Hold the Option key and choose Help > Report bug to start a support ticket.
Please attach a problematic PDF to the ticket. Thanks.
Done. Thanks for the quick reply.
Yes, I have discovered the exact same problem just a few days ago and are in contact with support via email about it.
By pure chance I discovered a note in DTTG where the PDF attachment could not be opened/read as usual. Like you I have many notes with PDF’s and this problem only occurs on some notes. So I did some testing and found this. I created notes from scratch on my Mac and sync’ed them normally to the iPad. So I found these problems
DTTG cannot read notes of the format Rich Text with PDF attachment
DTTG cannot read Formatted notes with PDF attachment
DTTG cannot read Formatted note with .jpeg attachment (but .png is OK).
This is true (on my devices, of course) for new notes. But there are also existing notes which display the same issue. It is hard to see a difference between the notes which have the problem and those who do not have the problem. I cannot say when this actually started from viewing the notes. Many notes of the problem types have been there long. So there must be something in the data that we users cannot see which causes this situation. For me it looks “random” on existing notes just like it does for you. But systematic for the new notes I created for testing.
It would be nice and helpful to hear if others can reproduce what I found. Perhaps a similar problem exists for other combinations of note type and attachment file type.