I tried this and the PDF documents act as they do in all other programs - opens Adobe Acrobat Pro. I set the file type declaration to open Adobe Reader but it didn't stick.
I will test this at home and see if I can duplicate.
A quick search shows that there are reported problems with Excel->PDF hyperlinks and Adobe Reader 7.0.
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I tested thi sat home and had no problems. I created a hyperlink to a PDF file, clicked on it and Adobe Reader opened in the proper resolution. I noticed that this was Adobe Reader 7.0, which was reported to have problems with opening hyperlinked documents from Excel 2007.
Sorry this doesn't solve your dilemma.