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!
There are a few different ways to add items to your library -
1. Find the item you would like to use in your web browser and use the browser connector.
2. Drag and drop a file into your library.
When the file has been added you can right click (control + click on Mac) and select "Retrieve Metadata for PDF." You may need to install a plug-in. If Zotero is unable to extract metadata you should instead select "Create a Parent Item" and then input the citation information manually.
3. Manually add the source.
Click on the green circle with a plus sign, select the type of source, and then input all of the information you would need for a complete citation.