IMAP is just a protocol for delivering mail. It allows storing your mail in folders on the mail server, etc. IMAP Idle support by both the mail server and the client is the equivalent of push for IMAP mail, but the Android mail client, which supports IMAP, does not use IMSP idle/push - it's set to poll your mail server for new mail on a regular schedule, the last that I looked.
K9 does support push for IMAP mail accounts.
There is probably a setting in mail that does not include the attachment when you forward. As should be obvious by now, I don't use the standard mail client.
Perhaps you need an app that displays PDFs. Can you copy attachments from mail to the SD card? There is an Adobe Reader app:
[ROM][10/20/2010]FlashBack21 v1 -Return to Factory Stock - UPDATED - xda-developers
I don't know if that runs on the Eris, but it does support GB. But, there are a number of PDF reader apps. Maybe one of these will work if the Adobe Reader app does not:
https://market.android.com/search?q=pdf&c=apps