No, I can't type in the text box, it only allows me to select from a drop down menu, which is the whole filesystem.
Anyway, I have found it, it was at /usr/bin/epdfview.
So it is all set now, but honestly I thought it would have been easier. I think that somewhere in PCLOS control centre there should be a section to set default file associations.
But then I try to avoid the terminal like the plague.
Thanks for your help everyone.