TL;DR – If you can't get Google Drive PDF links to load correctly as embedded PDFs when sharing them as external URLs in modules, change the last word of the public URL from view to preview.
I found that Google Drive hosted PDFs were not loading correctly when I posted public URLs for them as external links in modules.
The result was that students would click on a link for an external link to the Google Drive PDF in a module, and see this:
The white text at the top of this mysterious gray box links to the PDF, but the PDF does not load as an embedded document in an <iframe> the way it should.
The solution is to an an extra three letters to the url. For example, change this URL...
Doing this loads the embedded PDF correctly, like so...
For whatever reason, this view of Google Drive PDFs loads as an embedded document, but without adding the "pre" to the URL, it creates the blank gray rectangle of mystery.