Skim

Skim is a free, open-source PDF reader tailored for Mac users, focusing on annotation and research needs. It lets you highlight text, add notes, draw shapes, and bookmark pages—ideal for students, researchers, or anyone working with PDFs. With support for syncing annotations to Apple Books and exporting notes to various formats, it’s a versatile tool for organizing study materials or work documents. Its clean interface and customizable features make it a popular alternative to default viewers, offering flexibility without unnecessary bloat.