Can a PDF file be displayed on a dashboard page?

Did you try embedding it as an image within a rich text field on the dashboard?

This isn't really a great idea as pdf documents are typically a full 8.5" x 11" page and simply don't embed well. Nevertheless you can create a HTML code page referencing your pdf URL and embed it as a web page widget using markup similar to this:

<object data="https://eforms.state.gov/Forms/ds11.PDF"; type="application/pdf" style="width:100%;height:1600;">>
    <iframe src="https://docs.google.com/viewer?url=https://eforms.state.gov/Forms/ds11.PDF&embedded=true"&g...;
</object>

But you will probably have problems with the double scroll bars and need JavaScript to resize the <iframe> to match the size of the contents.

Does user need to interact with the file's content? If they do, attach it with the embedded code from the very service you use, like this for example https://ds11.pdffiller.com/ that will be different for sure, yet it depends exactly on the pdf's source

Will this work for a pdf in a file attachment field?

Kirk Trachy has a Magic buttons app in the exchange which manages to do a preview of a word or pdf file attachment on a form.  It also possible to embed a form of a particular record on a dashboard, so the combination of those two techniques may work.