I haven't had the problem myself but if anybody installed that Adobe app that is in the Store, that could be an issue.
Since the real Adobe Acrobat Reader cannot be installed on the ARM based tablets, the built in Reader app fills the job but in a limited fashion. The same goes with the Adobe app in the Store as well. The Adobe app should become the default app for loading PDFs at that point but sometimes it won't and there are times that it will crash. I tried it a couple of times myself and got rid of it after Microsoft added "printing" to the Reader app.
Some PDF files have features installed that the basic readers will not function. In that case, you will get a failure of it to open, but, Windows tends to pass the buck and located another program that will. That would be Word. And, on top of that, Word will probably error out as well since it is actually about 98% featured as the regular x86 version of Word.
There will be no solution for that other than downloading the PDF file with a different computer and then converting it to a ".docx" file.