Zotero is an amazing tool that should be in every researcher's toolbox.
You can use Zotero to import all of your references in the same place. Then, you can organize your references using folders, subfolders, tags, and notes.
When working with reference PDFs, Zotero has tools for you to be able to annotate and take notes in your PDFs and store them as notes in Zotero.
This allows you to make your notes easily searchable and well organized.
As a reference manager, Zotero also has the ability to cite your references in Microsoft Word or Google Docs. Using a reference manager allows you to dynamically cite your references so that it is easy to add references and change your reference styles.