To be graded properly on Forum, PDF documents must contain text and not just images of text.
To determine if a document contains actual text, copy and paste a section or all of the text into a plain-text editor such as Microsoft Notepad or Apple TextEdit. If no text is copied over, the selection was not actually text.
There are many ways to create a valid PDF. Some of the ones that usually work well include:
Write your paper in a Google Doc using the default Times New Roman font, then click to File > Download as... > PDF Document (.pdf)
Write your paper in Microsoft Word, then click File > Save As... > change the File Format to PDF.
Some that usually do not work well:
Creating a PDF by pressing Cmd + P and then selecting "Save as PDF" in the print menu.
These are not foolproof, particularly when using a lot of fancy formatting or images and sometimes cannot be avoided (i.e. for specific assignment formats). If possible, however, always check your PDF documents using the copy-and-paste test described above before uploading them to Forum.
As an instructor, it is helpful to be aware that, if in-text commenting and scoring on the Assignment Grader do not work, it might be due to the PDF not containing actual text. In this case, you can ask the student to reupload their document by creating a PDF as described above and then email email@example.com to clear the late turn-in.