Free up space used by Notes
Most likely media files take up most of the space. It would be cool, if you could access the attached images directly in the file system to compress them. Alternatively an option to compress or archive directly in the Notes App would be great. Unfortunately neither is possible in the current version of Apple Notes. Of course you can delete your notes in the Notes App. To free up space, you need to delete them afterwards from the "Notes App Trash" which is called "recently deleted".

Before you delete, I would recommend to export your notes and switch to a note taking app that stores media files separately. Further you could avoid attaching large images to Notes in the future and use a separate App for scanning and archiving documents.

Personally I use Bear for my notes and Scanbot to scan and archive documents. Both Apps make it easier to compress and archive larger files, since you can access and manipulate media files directly.

To my knowledge there is no perfect solution to export all content from the Apple Notes App. Though the free App Exporter might be worth a try, since it does a good job converting text and image attachments from the Apple Notes App into Bear compatible markdown format. Beware that Scans and PDFs are not exported by this tool, which might be a dealbreaker.