There was a post similar to this a while ago. It's possible to host PDFs on ArcGIS Online, but personally I don't want them to take up my limited space here. Therefore, I host the PDFs on AWS, though any web hosting would work, including SharePoint. (On the other hand, the last time I used SharePoint I remember with disdain the long, random URLs it assigned to things.)
Formatting the field is the easiest part. As long as it's a valid URL starting with "http" or (preferably) "https," then ArcGIS Online automatically makes it a live link. In some cases, probably for space, it replaces the link text with "View," which may or may not be desirable.
