Last week I installed Norton on my laptop which is running windows 10.
There's now a site a use almost daily which I am unable to open PDF files from, getting the message "Sorry, this file is currently unavailable. Please try again later. " I'm unsure if when installing Norton there is some type of security block now installed.
Website - Vehicles List • Central Car Auctions
Inspection Report for this paticular car - https : //www . centralcarauctions . com/i_folder/SD70EZZ/ vehicle . pdf
Like BJM I do not have problems on Windows 10 with Firefox in opening .pdf files but recently I have been getting the error message you mention when going to files linked to on screen ....... If I immediately go to open it again it opens OK. If I remember correctly the links are to other websites than the one I am already in.
FWIW
If it happens again I'll try to remember the details and post here.