adobe photoshop can do that editing...pero...very minor na kasi a PDF is already a "finish" product.
if you don't have adobe photoshop...then you will have to try other options...
editing that will only mean like converting it to a TIFF, BMP or JPEG or any other graphic file format para pede mo ng dagdagan or i-edit. after that you would have to save it back to PDF format (pero it might not be that clear minsan).
Pero kung you are just joining pages/adding pages of PDF format...kaya ito ng adobe photoshop plus a lot of other features..