how to transfer documents (PDF files) from pc to ipad ?

You can email pdf documents to your self, but you need to have an app on the iPad in which you can save them. Apple's iBooks app is one app that will work and there many other apps in the App Store that work with pdf files. Adobe Acrobat is another one and it is free to download.

This explains how you can transfer the files using iBooks.

Use iBooks with PDF documents on your iPhone, iPad, and iPod touch - Apple Support

You could install Dropbox on your PC and iPad, set up an account, then upload PDF from PC to Dropbox and finally use "open in" to download PDF to an app that can open PDF files.