Learn French: Personne n'aurait déchiffré ce manuscrit. - Nobody would have deciphered this manuscript.

In French news, the past conditional (aurait déchiffré) is often used to report unverified information. While it literally translates to "would have deciphered", in this context it means "Allegedly, nobody deciphered" or "Nobody is believed to have deciphered".

Why is there an n' before aurait but no pas after it?

The word personne (nobody) acts as the subject here, and it pairs with ne (shortened to n' before a vowel) to form a complete negative. Just like with rien... ne or jamais, you drop the pas when using this structure.

If I wanted to replace ce manuscrit with "it", where would the pronoun go?

You would use the direct object pronoun le. In compound tenses, object pronouns are placed right before the auxiliary verb, so the sentence would become: Personne ne l'aurait déchiffré. Notice that ne is no longer shortened to n' because it now precedes a consonant.

Does the past participle déchiffré ever change its spelling in this type of sentence?