Windows SECURITY WARNING for every link to another intranet PDF

This topic has been discussed many times, but I don't have a working solution from what I've read so far - another one of those "... it's worked fine for years, but now it doesn't" stories.

We have a longstanding intranet share that has several PDF 'jump documents' - each of which has links to many other PDF in various intranet locations. Up until the last couple of months (roughly) they've continued to work like a charm, but now the dreaded SECURITY WARNING is coming up each & every time one of the links to another PDF is clicked; ALLOW sometimes works, but not always - and responses are never remembered.

I've tried multiple iterations of other suggestions about configuring the SECURITY (ENHANCED) settings; turning off ENHANCED SECURITY; turning off PROTECTED VIEW; etc. - no change.

Can anyone offer current thoughts or suggestions to this vexing issue? It's incredibly tiresome for our users.