I have been using tablets for over a decade and one of the most useful activities has been marking up PDF documents with the pen. It's a great collaborative tool with the ability to ink notes right on top of a colleague's PDF document. It's also a great way to sign contracts sent in PDF format.
I love having a pen with my ThinkPad Tablet 2 and one of the reasons is the PDF Touch app available in the Microsoft Store. This Metro app takes full advantage of Windows 8 running on the tablet with a good interface that fully leverages manipulation by either touch or the pen.
Any PDF can be opened in PDF Touch for either review or editing. Once the document is opened it can be zoomed with the standard pinch/zoom gesture which is very handy.
The app is not a comprehensive editor and publishing system like some Windows legacy apps, but what it does it does very well. There are simple editing features like Draw, which supports marking up anywhere on the PDF document. It works very well with the pen and offers several different colors. Interestingly, the developers also support marking by fingertip or a mouse.
The Highlight tool works as expected, highlighting text by dragging the finger/cursor/pen over it. It also supports highlighting in several colors.
Other tools that PDF Touch supports is the Line and Rectangle features, which is exactly what it sounds like. You can add precise lines and rectangles with perfectly straight edges.
Once you select a tool such as Draw, a small menu appears in the upper right of the screen that offers simple settings such as pen width. They are touch optimized and can be easily worked with the finger.
PDF Touch is a great app for everyone, professionals and students in particular. It's an easy way to mark PDF documents with full touch support, making it perfect for indicating desired changes from colleagues. For a little fun you can print an email to PDF format, mark it up in PDF Touch, and then email it back to the sender. They'll go crazy trying to figure out how you wrote on their email.
A nice thing about PDF Touch is the price, which is only $2.99 in the Microsoft Store. It is easily worth that, in fact it probably paid for itself within five minutes of use for me.
Purchase: From the Microsoft Store