I’ve often been frustrated with nice PDFs that I wanted to quickly add some detail to or simply modify. Without forking out a lot of money for the Adobe Acrobat suite the only option until now was to take a screen capture and modify the bitmap.
Inkscape the open source graphical editing tool has just released version 0.46 which now has the capability to import PDFs. The PDF is converted to the native SVG format for editing purposes. You are then free to markup the PDF or modify it as you seem fit.
In order to try the functionality out, I loaded up a Fredericton Transit Route Map pdf from: http://www.fredericton.ca/en/transportation/resources/TransitMap13-.Brookside.pdf
I then modified the image by drawing a rough outline of what the service area for the bus route might be. I then saved it as a PDF. There are a few screenshots below:
Using Inkscape will be a good way to do quick PDF annotations. On a side note, OpenOffice 3.0 will apparently also have a PDF import capability.