PDFs typically are full of Postscript (except if they're just scanned images), which is just a text rendering language. As long as you keep the Postscript format valid, you could remove the watermark by just deleting that text.
I didn't know about PDFtk, but Ghostscript can take a PDF and turn it into text Postscript, and it can reverse the process.
ghostscript has always either rasterized or converted to individual strokes embedded fonts when I try that, but there's like 50 options for pdf->ps so perhaps I've got it wrong.
PDFs typically are full of Postscript (except if they're just scanned images), which is just a text rendering language. As long as you keep the Postscript format valid, you could remove the watermark by just deleting that text.
I didn't know about PDFtk, but Ghostscript can take a PDF and turn it into text Postscript, and it can reverse the process.