First of all, it is necessary to make it in bold - It is not always possible to compress the PDF document.
Compression depends on the context of the document, whether it contains text, images or images only.
PDF documents which contain compressed images can be compressed marginally.
Very often depending on the PDF format it can be released from unused objects, fields or annotations.
Text and images are the mostly compressed data.
You are free to use our other services to deal with PDF format.