There are many reference management software available in the market. Such as-
8. Reference Manager
10. Paperpile and so on...
The best open-source reference manager is Zotero. Although it's difficult to figure out how to get it set up just the way you want it. Because of the irritation of trying to figure out how to configure Zotero that led me to Citavi in the first place. Citavi lets you annotate the pdf documents. The basic difference, from my standpoint, is that Zotero is “large library” oriented, while Citavi is “writing project” oriented. You can use either for both tasks, but they have fundamentally different approaches. To the best of my knowledge, there are only two reference managers that integrate directly with google docs which are Paperpile and Zotero. Paperpile charges a monthly fee. On the other hand, Zotero is free to use and do charge a monthly fee for optional online storage (“sync”) of your files. Mendeley is free and gives you powerful tools to create and manage your references. It's flexible and works (and synchronizes between) all the commonly used operating systems and devices. It depends on you which software serves your purpose. Choose wisely and learn properly the integration of the software with your file.