How save PDF NOT editable in Illustrator

The "Keep Illustrator Editing Capabilities" setting for PDF files merely appends Adobe Illustrator data to the PDF. While it's common for PDF files to contain vector-based artwork it's also common for that artwork to not be edit-friendly. Some PDF files can be a real mess when opened in Illustrator.

If you want to send someone a PDF of artwork created in Illustrator but want to protect the contents inside from being extracted and re-used without your permission the best thing to do is rasterize the artwork before saving a copy of it in PDF. Just select whatever you want converted to pixels (individual objects or multiple groups of objects) then go to the Object menu and choose the Rasterize command. The dialog box will give you a variety of resolution and color settings.

Another tip: it's easy to "bake" watermarks or other visual "trash" into the elements to be rasterized. That would make it harder for anyone trying to live trace the pixel-based artwork. Just position your watermarks, patterns or whatever above the artwork. Select it all and rasterize it at once.

PDFs can be password protected at the Edit level and File>Open level. It's pretty easy for people to break password protection in PDF files that are protected at the Edit level. The protection is far more difficult to break at the File>Open stage, but it can be a big turn-off to clients if they have to fill in a password you provide just in order to view a PDF. I still think it's a good idea to at least password protect PDF files at the edit level since it sends a good hint the contents in the PDF are of value and not free to take or plagerize.