I have taught online classes since 2003, and back then we were often told to make our content into a PDF. That way it could be read by anyone, no matter what type of system they had. That advice has been handed down over the years and become canon, thou shalt post things as a PDF.
However, when it comes to document accessibility, we should not follow canon. PDFs are highly problematic for screen reader users. Some PDFs, like a scanned document, are not initially accessible to screen readers. Remediation can be done to these files, but it takes training and time. When training my student workers it takes about 35 hours to fully train them on PDF remediation. Documents initially written in Word and saved as a PDF have a higher probability of being read by a screen reader, but without some remediation it will still be difficult. For instance a user may have to listen to the whole document to get to the last page, rather than navigating there by using headers.
So what is the answer? If you are posting content in an LMS or on a webpage the best thing to do is post the content as html or as a page within the LMS. The next best thing to do would be to post the content as a Word file, attached or linked. Of course use the accessibility checker to make sure everything is accessible in the Word file. At this point in workshops I am often asked a few questions. The first is what about scans. Scans can be made accessible, my suggestion is to work with the library and they can put your scan on e-reserve and make it accessible for you. The second is, anybody can open a PDF, that’s not true with Word, is it? If you don’t have Adobe Acrobat you have to download Acrobat Reader. If someone doesn’t have Word they can download doc viewer from Microsoft. (Or PowerPoint viewer or Excel viewer) And for students on our campus, they all have access to Office 365. The third is, I don’t want anyone to edit my file, and they can’t with PDF, right? Actually they can edit your PDF, everyone on campus has access to Acrobat and if someone wanted to edit your PDF they could. But, you can lock your Word file to mostly achieve the same intended end. You can follow the directions from Microsoft on locking a Word file.
There is still a small place for PDFs. However, in most instances all of the same information can be shared in a webpage or a Word file.