Did you know that it is possible to edit PDF files on a Windows Phone? While there is no built in application to let you edit a PDF on a Lumia or windows phone, it is very much possible using a third party app.
Simply download and install an app called Xodo pdf reader and editor from windows phone store and you will be able to write directly on a PDF, highlight and underline text too.
Fill forms, sign documents
Not only that, annotating a PDF is also possible with others in real time. Xodo pdf reader also let you fill forms. You can even create a blank PDFs and use it to take notes.
The PDF files generated by it are compatible with Adobe Acrobat and other PDF viewers.