I've recently has some problems with Acrobat Reader which led me to install Foxit Reader making that my default pdf reader. Now, I 've resolved the problem with Acrobat Reader and managed to reinstall it successfully but I can't get it to be my default pdf viewer.
What should I do?

Depending on what OS you're using this may change slightly;

Right click any PDF document > select 'Open with' and click 'Choose Program'

Select Acrobat from the resulting list and tick the 'Always use this program to open this file'

Click OK!

Usually works for me!

(ON older Windows I think you can right click > properties to do it! )

Oh, and you could uninstall Foxit Reader if you're not using it now, that would help!

That worked for me too. Oh, and I had to uninstall Foxit Reader

Perfect. Thanks for the succinct and very helpful answer!