First of all, Readdle rocks. Everything the company makes is quality. If you need to print from your iPad at work, Printer Pro works really well. Spark is a really nice iPad and Mac email client, and Documents is a sweet little file manager. But if you only check out one of their apps, it should be PDF Expert, which is available on iOS and Mac.
I've been using PDF Expert nearly every day for a couple years, and it is simply the best PDF tool out there. Whatever you want to do with a PDF, this app has you covered. I'm sure there is something it cannot do, but whatever that thing is, I don't know and don't care.
I primarily use PDF Expert as a way to read and annotate PDFs, while keeping all my highlights and marginal notes in sync with other devices. I open a book and make highlights on my iPad, and whenever I access that file on my Mac, everything is in sync. I keep all my PDFs nested within one Google Drive folder, and I tell PDF Expert to keep that folder in sync. That's all there is to it.
PDF Expert works fantastically with the Apple Pencil so this means you can easily take notes in the margin or sign a document and send it off. You can save a signature and easily insert and resize it on your phone or iPad.
I really appreciate the tab system, which allows you to open two or more PDFs and switch between them easily. You can also set two PDFs side-by-side by simply dragging one tab to the side of the screen. Think of this as a sort of in-app split view.
Readdle released PDF Expert 6 last month, and if you're interested in what is new, check out Josh Voorhees post on MacStories.