"Save as PDF..." in Safari submenu

If you are asking whether it is possible to modify the Safari user interface, the answer is clearly yes, but if the question is whether there is a straightforward way to modify the contents of menus in an Apple-shipped program, the answer is sadly, no, not without deep hacking.

There are workarounds, however. A Safari extension could be written to add an item to the Toolbar or to a context menu, although not to the menubar. Writing that extension would be a developer question beyond the scope of this site.

An automator service that calls an AppleScript to save the current page as PDF could be added to the Safari menu (not the File menu) and assigned a keyboard shortcut.

As far as I am aware, however, adding a command to the File menu is not possible through the standard (i.e. supported) extension mechanism, and would require hacking the Safari binaries.