I discovered PDF-XChange over a year ago, after trying Foxit Reader and finding it had no plugin for Opera. If you search PDF documents you will find it signficantly faster than Foxit Reader, and more than five times faster than Adobe Reader.

You can customise it extensively — toolbars, menus, shortcuts, and searches. The browser plugin (for Firefox, Opera, or IE) can be customised independently.

Commenting and markup is extremely powerful, and you can add custom stamps too. Distance and area tools for scale drawings, transparent comments, and blend modes.