Please follow the enlisted steps to try and correct your issue:
Go to Start>Control Panel>Appearances and Themes>Folder Options. Then, click the File Types
tab in the context menu that opens up. There should be a list of file types associated with your system, scroll down till you get the .pdf file extension and check if it Opens with
Adobe Acrobat 9. If not, then change it to open with Adobe Acrobat 9, in the box below, and apply the changes.
If you have the .pdf extension already registered with Adobe Acrobat, but still cant open the PDF attachments in Outlook with the same, you should consider uninstalling and downgrading Adobe Acrobat to a lower version, as its latest versions are reported to have bugs working with Outlook Express.
There's an alternative: FoxIT Reader
, which I personally use instead of Adobe Acrobat Reader for opening .pdf files, as it is much faster and lighter on the system. I would recommend you to download and use it as well ..it should work fine with Outlook.
Hope it helps.