I have no problems opening pdf files with Outlook Web Access
and N800 OS2008 PDF reader.
When I tap the pdf-file attachment link in OWA
I get File download dialogue:

Name: pdf-file-name-is-here.pdf
Type: application/pdf
Size: unknown
Address: https://pathtomyexchangefolder...
Open with: PDF reader

Below that are Open, Save and Cancel buttons.

When I tap Open button PDF reader opens the pdf file
(no Evince here).

It just works.