Gaaiho Reader is a capable and feature-packed PDF reader.
Basic viewing is straightforward. Drag and drop a file, scroll through it as usual, use page thumbnails or bookmarks to jump directly to whatever section you need.
The program has plenty of zoom and viewing options. In particular, the horizontal, vertical and quad split tools display the current document in multiple panes, each of which can be scrolled or browsed independently.
There's a very complete set of annotation tools. You can highlight lines of text or an entire block, cross out or underline items, insert or replace text, even search for and markup the text of your choice (underline every instance of a phrase, say). You're able to draw lines and shapes, attach files or sounds, view comments and add your own.
You're also able to view, fill in and save forms, or export form data as required.
We were constantly surprised by just how much the program does. It doesn't just display comments, for instance: you can run searches, import or export them, migrate them or create a summary report.
The interface adapts to suit what you're doing, too. If you're in the middle of a Search operation and decide you want to import some comments, Gaaiho Reader doesn't complain or force you to close one dialog first. The Search dialog just fades into the background, and returns once the import is complete.
The program seems to be very lightweight. We tried opening multiple documents, and whatever the size Gaaiho Reader required far less RAM than Adobe Reader.
This efficiency does come at a cost, though. We also found that scrolling large documents from top to bottom took almost twice as long in Gaaiho Reader as Adobe Reader, maybe because it's not keeping as much of the file in RAM.
Gaaiho Reader is a likeable PDF reader with plenty of features which makes minimal demands on system RAM.