It just occurred to me that this is probably what it means. Often in Mexican law firms, there are law clerks, who have finished their degree courses but have yet to take the exam to defend their thesis. They are often given authorization in cases to receive and examine at documents or communications, so this fits entirely with your context.

I searched Proz again with the search string "pasante de derecho" to see if anyone had already answered this, and it turns out that Charles Davis has a previous (very thorough) entry for this exact same abbreviation, which I in fact had even agreed with (my memory must be failing), and it seems he even went through the same thought process ("por delegación")! It's often difficult to find previous KudoZ entries that only contain two letters.

Here's his previous entry:

https://www.proz.com/kudoz/spanish_to_english/certificates_d...

I have no problem if you want to close this without grading seeing as it's already been answered, although he has chosen a different route for his translation. This is the wording I normally use (after Becerra) because that's basically what it means in this context, i.e., an employee of a law firm who performs certain legal tasks but is not a practising lawyer yet.

The entry in J. Becerra's Dictionary of Mexican Legal Terminology that I've taken this from is as follows:

"pasante de derecho - law clerk, law student in his or her last year of school, though in practice many are offered low-paying jobs much earlier (even as freshmen) by law firms, notarial offices, courts and legal departments of banks and private companies, and students readily accept in order to gain practical knowledge of the profession for a few hours a day."