If you want to use an app to do this you can use the GoodReader app (From the app store) to transfer these files to the iPad over wifi.
If you don't want to use an app to do this you need to hookup the iPad to your computer. Click on the device and go to apps in the menu. Scroll down to sharing a pick the app that you want to using the document in. Drag and drop the document into the sharing window. When you are finished sync the iPad to iTunes. Before you do this read in the iPad manual about how to do this.
My favored approach for getting files viewible on the ipad is via Dropbox combined with Goodreader, Documents to go and Printbureau.
Goodreader, Documents to go and Printbureau share alot of functionality however none seem to do everything i want/need !
Combined with dropbox and the ipad ios4 open in command they make a very usable office suite !
You do need a couple of PC/Mac helper apps (Freely available)
PC/Mac based dropbox of cause Documents to go and Weprint server.
Dropbox is life saver app installed on your iPad ,primary PC/Mac and via almost any web browser your files can be accessed.
By the way Goodreader is NOT just a PDF reader, you can play video, music as well
For Powerpoint presentations you are going to need to buy the ipad Keynote app from the app store.
Be careful however because whilst it will open and run power point some complex formatting and effects are NOT supported !
Dropbox look here
Goodreader, Documents to Go and Printbureau search the app store.
An additional utility to consider adding is LogMeIn Ignition this allows you to remotely view and interact with your PC/Mac desk top is if you were right there , and yes it is secure.
This is in my view the current solution for using the IPad in a small business environment, short of expensive and complex labour (IT) intensive Citrix solutions.