How secure are password protected PDFs that are created with Preview?

The last Time I checked a 128 bit RC4 encryption of the whole document was used. Also, compatability should be vested.

Here's a screenshot of the output of GuaPDF:

But what does that actually mean and how secure is it?
Does this encrypt the whole PDF?
Is it compatible with Windows and Ubuntu?

Part of the PDF specification describes the encryption of PDF data, and Preview produces a 'to-spec' encrypted PDF.

Encrypted PDFs made by Preview are valid PDFs that can be opened (when the password is provided) by any PDF app that follows the spec on any platform.

The level of encryption depends on the PDF version.

Early versions of MacOS saved with PDF v1.4, giving RCA-128bit encryptions. Newer versions save as PDF v1.6, giving AES-128 encryption. You can see the version level of the PDF document in Get Info.