If the PDF contains just a pixel image all you can do is edit that image color, compression, res, and position. You could use Acrobat to edit via Photoshop but I'd just open it in Photoshop, edit, save as tif.
Maybe, if you have place around the line, you can draw a rectangle over the line what You'd like to delete, and give the new rectangle the color white and set it to knockout (no overprint). Voilá.