This is a mostly excellent PDF reader and annotator, superior to Preview in two main respects: (1) offering more convenient and rationally designed ways to annotate and edit PDF files, and (most signficantly for me) (2) offering tight-ish integration with PDF Expert for iPad (and iPhone), which allows, most of the time, reasonably seamless back-and-forth between the two platforms. PDF Expert for iPad is definitely one of the two best PDF readers-annotators for that platform, and there is nothing comparable to Preview offered by Apple for their iOS products.
My problem with PDF Expert for Mac at the moment is the fact that it continues to have number of longstanding uncorrected bugs that become really annoying if you are a heavy user and run into them regularly. In two-page view, Previous Page and Next Page stop working at random moments, and you need to switch into single-page view and back again to get out of the difficulty. At other random moments, it suddenly becomes impossible to highlight text to apply an annotation. At those times, switching to another page and returning to the previous page (if that works in the first place) gets you out of the problem, as does quitting and restarting the app. Notes one tries to write sometimes appear partially off-screen and can't be moved back on, except by deleting the note, starting it elsewhere and moving it.
Likewise, the "Readdle Transfer" feature should allow you to work on your Mac, then move to the iPad, then back to the Mac, or to another Mac — all the time synchronizing changes and opening the document to the current page. But it works only sometimes, and its behavior is unpredictable in many respects. Sometimes it detects an open document but just fails to sync, with an error message. Sometimes it doesn't detect the open document at all, even though it does detect PDF Expert as open on your other machine. Also, it is inconsistent in where it locates the document — so a PDF saved to iCloud on your Mac may open on your iPad as an iPad-only document, so your changes are not saved back to iCloud (and you might not learn this until it's too late). And other times it might work correctly, so you can't even plan a workaround, since you don't know what the app will do with any given file. And so on.
Now when the Mac app was launched (I was a very early adopter), the folks at Readdle were extremely responsive to bug reports and questions — even if the bugs were not always fixed. But more recently, they do not respond at all. The Twitter page devoted to PDF Expert, which used to be one place to get assistance when email failed, has not been updated in the past four months (not since December 19, 2016 — it's now April 23, 2017). The product itself does not appear to be dead — there was an update in March — but clearly the company's mind is elsewhere, probably on their email app Spark, and the imperfect but still promising product that is PDF Expert is being allowed to languish. That is unfortunate, and I hope Readdle gets back to work on PDF Expert, and turns some of its helpdesk attention back to PDF Expert users. This remains a product with great potential, and I still use it, but the daily annoyances will eventually get to me and other users — and I am keeping my eye out for something better.