To reduce the size of your PDFs, an operation sometimes essential to transfer a document to an online form, you can follow this short tutorial, or use a dedicated application. Lightweight PDF is one that just arrived in the Mac App Store and is free.
Simply drag the PDF onto the small window and the utility will compress it. The size of the file after the operation is specified. The gain is highly variable depending on the PDF and Lightweight PDF has no option to compress more or less depending on its needs.
The editor indicates that the application is designed for an optimal balance between quality and weight (bearing in mind that the more you compress, the more quality suffers). As a result, Lightweight PDF may not reduce the size of your documents sufficiently for some online forms or emails.
In this case, you can use the tools built into MacOS or a more advanced application, such as the excellent PDF Squeezer, which has many options, can batch process files, and integrates with the Finder, among others.