OS 10.8
Skim 1.3.21 (72)
Mail Version 6.0 (1485)
I have set Skim as my default pdf reader. When I double click on an attachment in OS X (10.8) Mail it gives this error:
The application Skim can’t open “birds_sample.pdf”. Do you want to save the attachment or choose a different application to open it?
That's Mail's problem. If you have problems with Skim not opening PDFs in general (not from a specific app) than report that. Otherwise it's a problem from the specific app.