[viggity]

It takes the embedded font out of your PDF, and then maps non-latin characters (japanese, cyrillic, etc) to render as if they looked like a latin character. So in the example on the site. "ӕ" will render as a "D" using my special font. And "ㅈ" will draw the "B" glyph. Then I do a replacement on the underlying text so all "B" are replaced with "ㅈ". It is more complicated than that, but that's the gist.

[bambax]

So basically it's a type of Caesar cipher where letters are mapped to something else one-to-one. Very easy to decrypt / reverse. If this tool ever became popular there would be hundreds of scripts to defeat it.

And as it is, it does not prevent "OCR", only copy-paste.

[smegsicle]

looks like it's actually one-to-many across unicode, if so then you could think of it as approaching one-time-pad encryption, with the key being the font

if the generator crafted a new font every time, never used the same codepoint twice, and kept the font separate from the document (pre-shared by being installed on the intended receiver's machine) then it'd be uncrackable!