PDF is the preferred format for documents in an assignment workflow. It's permanent, so you don't have to worry about your formatting getting lost in translation. Or, students accidentally or intentionally modifying assignment instructions. PDFs are also universally compatible, lightweight from a computing power standpoint, and very easy to make.

As a general rule: If the student doesn't need to edit the document, it should be sent as a PDF. Keep in mind that "annotate" and "edit" are two different things. If you post a PDF document with fill in the blank questions, students will be annotating their answers, not editing the question itself. If that document was in .doc format, they are able to edit the question.

Any document, spreadsheet or presentation can be exported to PDF, and anything you can print can be saved as a PDF through the PDF option in the lower left-hand corner of the print menu.

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When creating an assignment with an online, file upload submission, you also have the option of restricting the file type if you prefer to receive the submission in a certain format. This is not limited to the .pdf extension.