If i recall, the love affair between microsoft and adobe has dwindled, per se.

So presently, the headache free solution to read pdf's is with microsoft's edge browser. It can be said that its the new replacement of the readers. But the system goes a bit further as well with microsoft office products able to print doc's to pdf's

However, it is not to say that you cannot buy/own an adobe license and install the reader which would become your default.

For me, i have 3 licenses of different pdf readers creators convertors. But ultimately, i allow the edge to open pdf'f for reading due to its lightening speed of rendering/presenting the pdfs

In anycase, double check your win10 default programs. Everyone once in a while, the changes are altered - for whatever reason(s).  :-)

Thanks dbben but sorry I realise now I was unclear.

When I click on a link in Firefox which could be in gmail or on a website I have changed the settings that I want the Adobe reader to open the file however after a few days the setting in the browser seems to revert back to trying to download it?