Preview copies aliens. How to change the encoding/make it work?
The PDF file has been protected. You can’t copy-paste text from it. Plane 16 is displayed when a character is in the “private” area of the Unicode specification. This has been deliberately done to prevent you from copy-pasting.

Also, this is not OS X’s error character. It just appears when it encounters something in the private area of the Unicode spec.

A PDF file optionally contains a mapping to the Unicode private area for each character in the font. This particular PDF contains such a map, so it will display the correct glyphs only in a PDF reader.

Simple English: The PDF file is full of characters, which are mapped to different places in the Unicode private area. Normally, it would just display Plane 16, but because the PDF file contains a map which translate the private area characters into readable latin glyphs (a, b, R, F, etc), you see words in the PDF but aliens in other places.