I believe you need to leave the PDF entry off of your Level1Remove key. A PDF is not a Level1 attachment type, so you are actually making it a Level2 (with your referenced warning that requires you to Save). MDBs, EXEs, and LNKs are some common Level1s that you may need to change to Level2 with this entry.
You might also want to add the value “RemoveWarningFileTypes” to include commonly opened files (like PDFs, XLSs, DOCs, etc). This will give you the ability to uncheck the box to “Always ask before opening this type of file.”
Check out Regedit.com for more details.