This is a known issue in D2L. This issue is typically related to how the student created the document. If the document was created in Google Docs or Pages, then exported as a .docx, this can cause an error that only renders the first page of the document, even if there are more pages in the document itself.
The best solution is to have the student open the file in Microsoft Word, save the file, then upload it to the Submission folder. Students have free access to Office 365 and Microsoft Word, and we recommend using Microsoft Word to save and create files because it has the fewest issues with D2L.
This issue has also been ticketed with D2L under PRB0044124.