Amnoelle

I've been using this app for a very long time and I love it! It allows you to insert citations into your papers, but also to keep all of your PDFs organized. In the mobile versions you can annotate, highlight, etc. Newer PDFs will allow you to click on citations and download them to your library if you have access to them. It's an all in one app that works seamlessly. I've had a couple of small issues that I needed help with and customer support was super helpful and quick to respond.