Have you since solved your problem? (so that we know if you still need help)

Or if you did solve it, how did you solve it? (so that others can learn)

I still have the problem. When I attach a PDF file and double click on it to make sure that it is the correct file that I am emailing I get the error message, "There was an error opening this document. The file cannot be found". The same thing happens when I try and open the attached PDF file after I send the email. I also tried opening up a PDF file that was attached to an email that I sent months ago and I get that same error message. I tried using Adobe Reader DC, Nitro PDF version 12, and Able2Extract Professional 14 as the default PDF reader and it did not matter.

Open Tools/Options/Attachments/Incoming and click the drop-down for the Action for pdf content, Delete, restart TB, and double-click a pdf attachment. When the Opening window appears, click the Open with drop-down, Other..., then Browse... to the exe of the program that opens pdf files. Do not choose from the list in the Choose Helper App window. Check the 'Do this automatically...' box. Do you still get the error message?

Thank you for the instructions, but that did not fix the problem. I tried using both Adobe Reader DC and Nitro Pro 12 as the default application for opening PDF files.

I wonder if this is related to the bug with opening attachments that have spaces in the file name:

https://support.mozilla.org/en-US/questions/1274716

You are correct. If I try sending a PDF file where there as no spaces in the file name then I have no problems opening up the PDF file either before or after sending the email.