Convert PDF files into pictures (JPG, BMP etc...) or into PDF files with only one picture of the content per page. This way nobody can modify your pdf files because there is no text information. It is like you would printout the pdf file, scan it and save the scanned picture as pdf file. This is the only way to protect the file against changes (you can not use our tool PDF Editor to edit the text then because there is only a picture and no text information).