Right click it and choose Get Info
If you see Preview in the "open with" list, choose it.
If not choose "Other" and then navigate to your Applications folder and choose Preview
In the same section, under Use this application to open all documents etc.
Hit the Change All button
I just upgraded to Lion. Now when someone emails me a PDF file, it is converted to ascii. I can't send or receive PDF files. This is also happening with Pages when I try to email pages. Does anyone know how to set up mail so this doesn't happen?

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