If you double-click a .pdf file icon in an email, the file is supposed to open up in a viewer such as Adobe Acrobat Reader, or in a web browser. If this doesn't happen for you, there are a few potential explanations and solutions. You may need to reset the default program for .pdf viewing. You may need to sever an old default association. Or you may need to update your version of Adobe Acrobat Viewer.
Check that you have Adobe Acrobat Reader installed by trying to open a .pdf file already stored on your computer. Find a .pdf file in Windows Explorer, right-click on it, and select "Open." If the file doesn't open, then search Windows Explorer for files with the name "Adobe." If the Acrobat Reader program doesn't come up in your search, you'll need to download it at the link provided in the Resources section below. Make sure to download the version for your operating system. If, after installing Adobe Acrobat Reader, you still can't open a .pdf file from an email, try the next step.
Associate Adobe Acrobat Reader with .pdf files. The problem may be that Adobe Acrobat Reader isn't your default program for opening PDF files. Right-click the .pdf file in the email, select "Save as," then save it to your hard drive. Look up the file in Windows Explorer, right-click on it, then select "Open with." Find Adobe Acrobat Reader in the file tree that pops up and double-click it. Check the option to remember your selection. This will associate .pdf files with the Adobe Reader in the future. If this doesn't solve your problem, go to the next step.
Reinstall Adobe Acrobat Reader even if you already have it. The trouble may simply be that your version needs updating. For example, version 5 of Acrobat can't open up a .pdf file saved in version 9. Use the first link in the Resources section below to download. Make sure to download the version corresponding to your operating system. If this doesn't solve your problem, go to the next step.
Save the .pdf attachment to your drive and open it from there. If you can do this, and you have Vista and use Windows Mail, then according to winhelponline.com you should try the following. Download unassoc_1_4.zip to your Desktop from the last site listed in the Resources section below. The link on that page to download the file is not prominent. It is about three inches above the start of the comments section. Extract the contents to a file with your computer's unzipping software. Run unassoc.exe. Select ".pdf" from the file-type list that pops up and click "Remove file association." Now right-click on the Adobe Acrobat Reader shortcut icon (should be on your Desktop) and select "Run as administrator." Open Adobe Acrobat Reader, open the Help menu, then click "Repair Adobe Reader Installation."
Try the following if you have Firefox and are getting a message like "cannot be used to view PDF files in a web browser." Close Firefox. Delete "nppdf32.dll" from ?"c:\program files\mozilla firefox\plugins." Search for the file "nppdf." It will be in a folder address like "c:\program files\Adobe\Reader 8.0\Reader\Browser." Copy it to the plugins folder where the "nppdf32.dll" file was. Open Firefox again.