Zotero is a great open-source tool that allows you to keep all of your sources in one convenient place! Using Zotero's easy interface, you can create folders to store project-specific PDFs, book chapters, online sources, and more.
Zotero also has a convenient browser extension that lets you save PDFs, webpages, and other sources right from your browser window!
You can download a desktop version of Zotero. Below is what the desktop version looks like:
Zotero also has a browser extension that's compatible with Chrome, Firefox, and Safari. Once downloaded, you'll have an icon on the top right hand side of your URL bar in your browser window. It should look something like the icon circled in red below: