I can copy Excel and PDF files to my card (I made a new directory on it), but can I open those files from my phone?
If so, how?

You need to purchase a third party app to view those types of files. Like this:

MasterDoc - Picture Perfect Document Viewer, no subscriptions, no fees, no limits! : BlackBerry 8820 : Handango

Or, you can email them to your BB as an attachment and the BB will be able to view them that way.