pdf link problem

I am suddenly having trouble with the pdf links in EN to my
publications
file. For ~ 20% of my links, I get a message stating the
"file name"
could not be launched. I can clear the pdf link, relink it
and all is
fine, but I hope that there is a better (more global) method
to fix this
problem? 


Thank you!
Cyndi 

Cyndi Swanson
Biotechnics