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You might be able to open the file in an object oriented drawing program like Illustrator, Affinity Designer, or a PDF reader with limited editing capabilities, like PDFPen. Unfortunately, none of those are free.

Alternatively, if the watermark doesn't cover anything important just open it in Preview and put a white box over the watermark.
 
If the watermark was added to the file when it was a JPG, then converted to PDF, it's probably near impossible to remove it.
 
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