I have observed the following issue on my wife's iMac.

This is a mid-2007 iMac (10.10.1 Yosemite) running Outlook for Mac 2011 update 14.4.7.

Since upgrade, in about late Oct 2014 (from an older OS and the 2004 (?) version of Outlook), most (not all) MS-Word and PD|F attachments cannot be opened when received via Outlook. For instance Word files cannot be previewed or opened, either from within Outlook or by saving to local disk. The corruption is usch that even the 'Recover all text ' option does nothing. Nor do any of the 'recover from corrupted files' options improve the situation.

Further even when the (downloaded) attachment is accessed via MS Word 2013 on a Win 8.1 PC then the same corrupted file behaviour is observed.

Likewise if the e-mail message with the attachment is forwarded to a PC and accessed via Web Mail (gmail via Firefox), then again the attached file is found to be corrupt and cannot be opened.

The same situation applies to PDF attachments.

Initially I suspected a fonts issue on the iMac, and went through removing duplicates and clearing font cache. This hasn't changed the situation.

Yet all these same attachments can be opened when received via  Web Mail (iCloud accessed via Safari on the iMac and opened with Word 2011).

Looking at the above : the mail server (mac.com) cannot be at fault since the files can be accessed via web mail. The version of Word cannot be at fault since it will open the version of the file accessed via web mail fine.

My only conclusion is that Outlook is somehow corrupting the attachments as they are downloaded from the mail server. It is possible that the fonts issue (which I only addressed on the 2/Jan) will resolve the issue, since although it doesn't change the problem for already received attachments, it may work fine for new files.

Does anyone else have same version of OS and Outlook, if so, please can you report if any issues with MS-Word and/or PDF files (or indeed other file types).