If you're using Outlook Express check to see if the following appears at the top of your email: "OE removed access to the following unsafe attachments in your mail: yourpdf.pdf"
If it's there, its basically a security doodah in Outlook Express that's there to prevent you infecting your computer. A pdf file will be lsited in your PCs Internet Explorer 'unsafe file' list and any e-mail attachment with a file type that is reported as "unsafe" is not downloaded, and access to such attachments is always removed.
If you want to always be able to view such files, do the following to allow Outlook Express to show the attachment, but be aware that you will be making your PC somewhat less secure in doing so.
On the Tools menu, click Options. Click the Security tab and under 'Virus Protection', uncheck the 'Do not allow attachments to be saved or opened that could potentially be a virus' checkbox and then click OK. Job done.
Your pdf files should now be viewable!