If it is an electronically signed PDF, don't compress it: the signature is lost.
How to do it with Requisito Listo
Open «Reduce PDF or photo size».
Pick the PDF or photo.
Choose the maximum (500 KB, 1 MB, 2 MB, 6 MB…).
The app finds the best quality that fits. If it was already smaller, it leaves it as is.
At the end you'll see a checklist: size, file weight, background and format. If something doesn't comply, it tells you what to fix before the portal rejects it.