Go to the Design Tab and make sure you switch to the Page View (look the bottom right corner of the screen)
Once you see the Page View (used to see what the PDF will look like for your response) scroll down and you might see which object is being trucated (it will be covered by a red rectangle). This often happen if you have a element that is too big to fit on one page (like a single or multi choice field).
You will need to adjust your element so that nothing is trucated.
this helped thanks! There must have been a change made-I have always been able to download as .pdf even when it was trucated. I just put the text in a few differnt text boxes, which did the trick.