In SpeedGrader, if a PDF file is submitted with a very large page size (i.e. the physical page size in inches/mm set in the PDF header), then SpeedGrader is not able to zoom out far enough to see the entire page. The "-" zoom button becomes grayed out before the whole page is visible.
Some tablet and scanning apps seem to produce PDFs with a large page size, which triggers this. (In particular, the popular CamScanner app does it if the page size is set to Auto Fit.) It is obnoxious to try to grade these, since you have to scroll back and forth to read each line.
External PDF viewers (e.g. Apple Preview) do not have this problem and are able to zoom out just fine. So downloading and using an external viewer is a workaround, but not convenient if you are also trying to add comments or grading markup to the submission.
Alternatively, try to convince your students to fix the PDF settings on their scanner or sketch apps. But that could be hard to do if you're not familiar with what they're using.
The attached PDF was created deliberately in Acrobat with a page size of 100x150 cm, but I've seen documents from actual scanning apps that were about this size. Try uploading it to an assignment from a Test Student view, then try to grade it in SpeedGrader, and you should see the problem.
