Added function to export the annotated PDF.

[somefoo]

I added the function to export the annotated document as a new PDF.

I am aware this isn't the perfect solution, but I believe it is a nice workaround for now to get an annotated PDF :)

It uses pdf-lib and some additional functions in annotation.js.

[Asakamizu]

I suggest allowing annotation to be exported as FDF or XFDF, which enables the user to import the annotation into the original pdf file in Acrobat to save the annotations made in the extension.

[somefoo]

Agree this would be a nice addition. This could be an option in the already existing screen used to store the annotations? In fact, it may be a good format to store in every time?

Replacing your existing PDF with the PDF stored using the pull-request approach will have the same effect and is more portable (it doesn't require Adobe Acrobat, which is not available under Linux per default).