It all depends on the app with which you want to read them with.
The easiest way is to have the app installed on your iPad and then connect the iPad to iTunes. Once it's connected, go to the "Apps" tab [for the ipad]. Scroll all the way down to the bottom of the page, locate the app you want to use and click on it to highlight it. Once it's highlighted, you can drag-n-drop PDFs into the box for the app.
You could also e-mail the PDF and, once the e-mail is opened, press and hold the PDF attachment, you can elect to "Open In..." the app you wish to use.
There is also cloud storage (like Dropbox or iCloud) that you can access the PDF to "Open In..." your app.
Hope some of this helps.
Marilyn