setting handlers to progs on stick

OK, since there might be some issue when running mime types calling up external programs placed on other drives, I tried to set in portable TB progs which are on my stick.
So for jpg and doc (open office) etc. When it came to pdf however, I was allowed only to use the sumatra on stick explicitly selecting it, but the tickbox where one can say it should use it from now always is grayed out iin this case and can not be selected. I would have to manually select sumatra each time.
Why is this so? Why can I select for jpg and doc and others some progs free but not for the pdf?