Skim is a free and open-source PDF viewer for macOS. It[SQ]s known for its simplicity, speed, and focus on functionality for researchers and students who need to work with PDFs extensively. Key features include :

*

Annotation Tools:

Allows for highlighting, underlining, note-taking, and adding comments directly within PDFs.
*

Sync with External Note-Taking Applications:

Can be integrated with external applications for more comprehensive note management.
*

Multi-Column View:

Helpful for viewing PDFs with two or more columns of text.
*

Multiple PDF Management:

Can handle multiple PDFs simultaneously, enabling easy comparison or studying.
*

Outlining and Navigation:

Provides clear outlining and navigation tools to quickly move between sections and pages.
*

Lightweight and Fast:

Unlike heavier PDF viewers, Skim is designed to be resource-efficient and quick to load.

While it lacks some of the bells and whistles of commercial PDF readers (like Adobe Acrobat), Skim excels as a dedicated and powerful viewer for academic or professional use focused on reading and annotation.