This topic has been reported many times throughout the Internet, with the 'Insert PDF' menu greyed out both in the main menus and the contextual menu despite the container field being set to interactive content within Windows.
The acknowledged solution to this is to install Adobe Acrobat Reader DC and all's fine.
However, we had a problem earlier this week on a Windows 2019 RemoteApp server that was streaming both FileMaker Pro 19 and FileMaker Pro 15 (until we can finally migrate these few clients to version 19). We believe this could also happen on Windows 10 or 11, hence wanting to document the solution for future reference as we could find not find this particular solution on the web.
We'd uninstalled and reinstalled Adobe Reader, restarted and tried various other ideas, including setting up a new local single container field test database. All was fine with FileMaker 19 but FileMaker 15 continued to display the greyed out menu.
We have 7 of these streaming cloud servers consisting of Windows 2012, 2016 and this one was our second 2019 server. We have very strict procedures to ensure all servers are configured identically, but this was the first time we had this problem.
We finally resolved the problem by uninstalling the Adobe Reader 64-bit version and download and install the 32-bit version. After this, FMP v15 displayed the 'Insert PDF' menu and version 19 continued to do so. We will revisit this to see whether we can run both 64 and 32-bit versions on the server, but have to wait until weekends before we can do so.
Finding the 32-bit version of Adobe Acrobat Reader DC is via a link at the bottom of the Adobe download page, after which both the 64 and 32-bit versions are displayed using the filter fields as shown below
I hope this helps someone in the future avoid the amount of searching and testing that we had to do.