Something to keep in mind regarding Outlook searches and additional mailbox. The comments below are in the context of Outlook 2010 and Exchange 2010 as that’s what we use at work.
There’s two different search providers that come into play when searching via Outlook. They are used depending on how the mailbox is accessed by Outlook.
If the mailbox is accessed in cached mode (so the folders are actually in an OST file on your computer) or if you are searching a PST file, Outlook uses Windows Desktop Search (WDS) to do the search. WDS is set to index many locations on your computer and if there are OST and PST files in Outlook it indexes these too. You can tweak WDS’s options by clicking on “Search Tools” and then “Search Options”. Click “Indexing Options” in the window that opens to change the locations and file types that are indexed.
However, if the mailbox is accessed in online mode (so there are no OST files involved) Outlook passes your search query to the Exchange server who then returns with results.
This distinction doesn’t usually matter except when searching attachment contents. In that scenario WDS returns more results as it is able to index more file types. The Exchange server in contrast has a lesser number of default file types it can index and it is up to the Exchange admin to install filters for additional file types.
At work, for instance, all our users access their own mailboxes in cached mode – so search results there are more comprehensive when compared to search results from additional shared mailboxes that are accessed in online mode. The PDF file type is not included in the default file types for Exchange 2010, so shared mailbox searches don’t return emails with PDF attachments even if they contain the search term. Excel and Word file types are returned in the results though.
This is a good link to read for more info.