What free software can I use to read any PDF file I want on my iPad?
Personally, I use iBooks for my PDF storage and viewing. It's already on my iPad to view iBooks I have bought, and you can send a PDF from any other application to it.

Having said that, Mobile Safari will also display PDFs if you follow a link to them.

I also use DropBox, which will display PDFs, amongst other file types. The bonus there is that you can then use DropBox on your Mac/PC/Linux to put the PDFs where you will be able to access them.