Allie put a description of her cat on a poster and put them up around the neighborhood in hopes of finding him.
I regard the above as odd-sounding, especially the bolded parts and, I intend to reword it as the following. Correct me if I am wrong. Thanks.
Allie made a description of her cat on a poster and put it up around the neighborhood in the hope of finding it.
The problem with the sentence is "a poster" and "them". If you change "them" to "copies of it", the sentence makes sense. ""put a description" is ok. Even better, "Allie described her cat on a poster..."
"in hopes of finding" is ok, but "in the hope of finding" is probably better.