Your answer is in the blog post I just linked to. Currently WooStamper is the only plugin for WordPress which will preserve your PDF *as is* yet also add a mark and/or password protect/encrypt/protect it. Check out the link please.
From my internal test, I was able to remove the PDF password from my PDF easily using some online webisite.
Then I can open the PDF with Adobe Acrobat PRO, and delete your watermark text manually.
All that said, most the paid watermarking plugins for WordPress PDFs WILL encrypt PDFs (up to AES 256 if using WooStamper, for example)… and this would have helped a lot to prevent what you just accomplished on the web.
But, are you able to remove the WooStamper AES 256 encription?
I mean, I’m easily able to remove password I created with Adobe Acrobat PRO using one of the million website available, which is crazy in my opinion.