> Shouldn't those parenthesis be escaped in some way? Or is it the validator parser which is buggy?

The spec allows nested parenthesis:

PDF Spec 1.7 3.2.3 Literal Strings

A literal string is written as an arbitrary number of characters enclosed in parentheses. Any characters may appear in a string except unbalanced parentheses and the backslash, which must be treated specially. Balanced pairs of parentheses within a string require no special treatment.

Example from the spec:

(Strings may contain balanced parentheses ( ) and special characters (*!&}^% and so on).)



So I bet the validator is buggy.