How to remove watermark from pdf using pdftk?

Just a little add-on to Dingo's answer as it did not work for me:

I had to first uncompress the PDF document in order to be able to find the watermark and replace it with sed. The first step involves uncompressing the PDF document using pdftk:

pdftk original.pdf output uncompressed.pdf uncompress 

now, the uncompressed.pdf can be used as in Dingo's answer:

sed -e "s/watermarktextstring/ /" uncompressed.pdf > unwatermarked.pdf

I then repaired and recompressed the document:

pdftk unwatermarked.pdf output fixed.pdf compress

very simply task to perform:

use sed:

 sed -e "s/watermarktextstring/ /g" <input.pdf >unwatermarked.pdf

but, after, be sure to repair resulting output pdf

pdftk unwatermarked.pdf output fixed.pdf && mv fixed.pdf unwatermarked.pdf

all into one command:

 sed -e "s/watermarktextstring/ /g" <input.pdf >unwatermarked.pdf && pdftk unwatermarked.pdf output fixed.pdf && mv fixed.pdf unwatermarked.pdf

text watermarks are nothing else than a text between two tags inside the pdf compressed code