Why would anyone need a pdf editor on mac when you have a macOS Preview application? Preview is the default macOS viewing platform for many types in addition to PDFs including BMP and JPEG files.
Because, as you said, Preview is a viewing platform...
While it's certainly more capable than its name suggests, Preview does not have actual PDF editing capabilities.
Good job.
Do you use any PDF library for this?
Also, why not using Swift? I believe this would definitely increase the chances of the project to last longer (& get more people working on it)