The reference manager I would personally recommend to everyone is Zotero. It's free, open source, and highly customizable. In this post I'll give a basic overview on how to install and use zotero, including a PDF with a step-by-step guide, and links to add-ons you can install.

This post will be updated later to include more details and links.

Storing and referencing the sources you use is something everyone will need to do at some point in their research. You can just download pdfs to your PC and write your citations manually of course, but there are a ton of options available that automate (parts) of this process. Lots of people will be familiar with Mendeley for instance, or maybe you prefer online services like MyBib or EasyBib. You might even be writing entire BibTeX files by hand!

Zotero will take make doing all tasks related to managing and citing your sources a lot easier; especially once you set it up to work the way you like.

Zotero

zotero.org

A free & open source reference manager for all platforms with integrations for all modern browsers & text editors

Extremely customizable, lots of amazing plugins, can be completely tailored to your wishes

Powerful options like automatically grabbing/saving/renaming PDFs, fetching uniform metadata, and more

Useful Links

Slides that walk through setting up Zotero including useful plugins

List of Zotero plugins (a subview of the main information tool list that can be found here)

Looking for more tools to improve your research workflow? Check out the main list of information tools on this site for a lot more.