In Nintex Form, when I click the "Print to PDF" option, the "Converting to PDF" modal window comes up, but PDF is not generated and the modal overlay doesn't go away.
I'm not saying this is the problem but maybe something to check. I had a similar problem where I could print the PDF but my users couldn't. They just had a blank screen similar to what you are describing.
It ended up being an image in the Nintex form to which my users didn't have read access and so the PDF could not be generated and somehow gets stuck. Did you rule this out? Is there any images that might display as a x on your form?