There are several ways to shrink pdf file size without completely compromising quality. Remove blank or unnecessary pages, reduce the resolution of large images, and avoid embedding excessive fonts or multimedia. If the PDF was created from scanned pages, optimizing those images can have the biggest impact. You can also use a PDF compression tool to automatically optimize the document. It is a good idea to save a backup of the original PDF first, so you can compare the compressed version and restore the original if necessary.
Source: https://dataqix.com/how-to-reduce-pdf-size/