Instead of using PST files per-se you can use a regular Exchange mailbox, or "archive mailbox", and then apply a retention hold to that mailbox. The result is an online PST that you can't delete messages from.
Alternatively, create the mailbox, and then set permissions on all the folders to "read only". Users can browse, but never edit or delete the messages.
To set permissions in bulk on E2010 SP1 and newer, use Exfolders. Older releases should use PFDAVAdmin. Use this tool to set a public folder hierarchy or mailbox to read only for users that need access.
Your best approach is to use a mail enabled DL, and use that to apply readonly permissions to the mailbox, and then add users to that DL.