Of about 30 users, there is just one who isn't able to view PDF documents. This is in a container field set up for external storage. This user is on a Windows 10 machine (as are many others) using FMP 16v4.
Before the issue, she had never previously opened Internet explorer, so I had them install a PDF add-on into IE. They used the Foxit viewer add-on, which should be fine since PDFs can now be viewed within IE. They double-checked that the extension is enabled on IE, and that ActiveX was set to work and not be blocked.
Any thoughts on this? I'd say there is a firewall blocking things, but (1) no other users have issues and (2) it's a container field (not a web viewer).