FI don’t know the why, but here goes an old trick that used to work. Try creating, also pn your desktop, a small pdf file and name it just like the unerasable one, using copy and paste if needed. You should be asked if you want to replace the exising one, answer yes. Now you should be able to erase the new file. If you have difficulties in creating a new pdf, use a COPY of any existing one and rename it. Luck.
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