Most people are familiar with PDF files and part of the reason that they are so popular is because, reassuringly, files stay the way that the author intended. The font and layout and everything about the document do not get adulterated when they are opened on another computer in the way that other document formats can do. For that reason they are popular for CVs, contracts and similar documents. PDFs are also fairly compact files so they are convenient for downloading too. Anyone can create a simple PDF file from Word, Pages, Keynote etc on their computer or iPad. Lots of apps can read PDF files with Adobe's Acrobat Reader (for Mac, PC and iPad) being the one most people will have heard of.
Desktop software for editing PDF files varies but can be pricey. If you want the full functionality of Adobe Acrobat it will cost £13 per month (Adobe have moved to a subscription model for their software) but you may be interested to know that you can also edit PDF files on your iPad for a fraction of the cost. Variously these PDF editors allow you to edit the text of a document, fill in fields, and write annotations (just like pen and paper I hear you say, and you would be right except that you are saving paper this way) and some let you create PDFs too! There are a lot of apps to choose from and they all have their various strengths and weaknesses. Further down I have suggested a couple for you to try.
Before you look at any apps, you might be wondering why I am mentioning these to you..... and the main reason is marking and a secondary reason is taking notes. A PDF editor will let you take a PDF file and draw and write all over it and send it back again. It is a tool for the paperless environment with the plus that both your and your students can have a copy.