I have not seen these files, so I do not know what type of watermark there is. Genereally, if I was looking to 'Securely' sanitize something I would do the following:
1. Print out the information.
2. Scan the information back into a computer system.
3. OCR all of the text and ensure that all spelling is correct, there is no odd punctuation, all bold/italics/font sizes are in places that are approriate and make sense.
4. Place images back into document, or better still recreate the images.
5. Create PDF of new sanitized workbook.
That is a lot of effort, but it is the best way I know of to handle these types of situations. Best of luck!