Is it maybe a solution to save the PDF in the temporary folder?
Thank you for you post!
You might start by checking FileMaker Pro Send Mail compatibility issues
From there I'd recommend testing a New File to see if it is persistent across different solutions, or if maybe there is something going on with you particular Solution. You also might verify any Plug-ins being used are up to date and compatible with FileMaker 15.
I'm also going to move this thread from the FileMaker Community Feedback Space to the Discussions Space where others might find your post easier and offer further advice.