When the verb is at the beginning of the sentence, the pronoun goes after it. You could also say "eu o encontrei", but it's not that common in Portugal.
erudis has already answered to this. "When the verb is at the beginning of the sentence, the pronoun goes after it". Some other examples "Disseram-me muitas coisas" / "Caçaram-me na floresta" / "Cansei-me de caminhar".
Here in Brasil we don't use this structure so much in spoken language, so it looks like something written, not spoken. The most colloquial way to say this sentence here is "Encontrei ele trabalhando no jardim"