https://github.com/pdfcpu/pdfcpu
https://pdfcpu.io/core/watermark
pdfcpu stamp add -mode text "Hello World!" "op:0.2, s:1.0 rel" in.pdf out.pdf
pdfcpu stamp add -mode image "my_pic.jpg" "op:0.2, s:1.0 rel" in.pdf out.pdfhttps://pdfcpu.io/encrypt/decryptPDF
pdfcpu encrypt -upw upw -opw opw decrypt.pdf encrypted.pdf
pdfcpu decrypt -upw upw encrypted.pdf decrypted.pdf