Assuming you do own right to documents, then it is simple work on Watermark On Pdf . Go to Edit PDF, And that all there too it. Likewise, you can do it on Watermark On Pdf to remove textbox once you go to Edit PDF within Watermark On Pdf . If you are asking about locked content or copyright contents you do not own right. Then you can not ask this type of question here.
Again if you have any problem, please write an email to DMCA watermarkonpdf.