Canvas can accept any type of file for an assignment submission, however in order to display work within SpeedGrader for marking it must be from a specific range of common filetypes. SpeedGrader can also display other submission formats such as video, images or websites, but for submitted documents it will only accept documents such as Microsoft Office filetypes (.doc, .docx, .pptx etc.), or Adobe Acrobat pdf.

For a full list of those file types that are presentable in SpeedGrader see: What file types can be previewed in Canvas

A common problem with files that students have submitted is where students have submitted files created and saved in the Apple 'Pages' file format. You can verify if this is the case by attempting to view the submission in SpeedGrader, and seeing within the Submitted Files panel on the right a file with the '.pages' extension.

A way to prevent this is to use the 'Restrict upload file types' setting when creating the assignments. This prevents studens from being able to upload anything apart from these filetypes, and encourage students to instead save as a 'pdf' type file. For more information see: How do I create an Online assignment

Students are advised on the assignment submission screen what file formats are acceptable, and it may be good practice to advise students on this elsewhere within the module resources. Most desktop application now capable of saving to PDF and it might also be worth highlighting to students that they are entitled to free Office 365 installations for Mac or PC: https://www.liverpool.ac.uk/csd/office/