Can you please post the error that you are receiving when you try and open the .pdf file? Also can you open other file types? Ex: Jpgs(pictures), .wmv(movies), .mpg(more movies). Another thing. How are you accessing your email? Are you using a mail client such as Microsoft Outlook Express or Microsoft Outlook, or are you using a webmail client by visiting
www.yahoo.com or similar and logging in through there? If by chance you are using microsoft outlook express you should make sure the setting that is notorious for blocking file attachments is not turned on. It can be located by clicking on the Tools menu and then clicking on Options. Once the Options dialog box is open you will need to click on the Security tab. Then under the Virus Protection section ucheck the box next to "Do Not allow attachments to be saved or opened that could be potentially be a virus"
If those things do not resolve your problem please post the error you receive when trying to open the file and I should be able to get you in the right direction.