Security Method is grayed out

I need to change the Security Method on a PDF file so I can reduce the file size with file optimizer.

The problem is the drop-down menu for Security Method (File > Properties > Security) is grayed out. Also, the button for "Change Settings" and "Show Details" are grayed out as well. Below, the Document Restructions Summary shows "Allowed" for everything listed.

In the meantime, "PDF Optimizer" remains grayed out and I need to reduced the size of a PDF.

Any help with this would be appreciated.

Thanks