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Is there way to protect PDF so others can't use online tools to unlock PDF?
Or whats the best way to secure PDF so others can only see it, but not edit?
No protection is 100% secure.
It's not possible to view a PDF without downloading it first. If you view it "online" it's either saved to your computer as a temporary PDF file (which means it can be copied, edited, shared, etc.), or it was converted to a different format.
No matter how complicated the password you set to protect your PDF, there are always some tools that can crack it, I think.