What's a good PDF score reading app for iPad? As a chorus and opera singer, I frequently download public-domain scores in PDF form and read them with my iPad. There is a built-in PDF reader on the iPad, and it is serviceable for music performance. But there are flaws:


Some PDF files containing high-resolution scans of the score take a long time to display. The degree of lag is a function of how the PDF file was made. In some scores it seems like the page starts off blurry and comes into focus. 
It's difficult to jump to arbitrary points in the score (e.g. director says, "let's skip to page 166", those with books turn easily, I fumble fumble on the iPad)
It's hard to add bookmarks to PDF files to correspond to rehearsal numbers, to speed up jumping to points in the score.
There's no way I know of to make annotations on the score during a rehearsal, even an "x" to say "review this later".


A good PDF score-reading app for iPad would fix these problems:


App should read ahead in the PDF file, digesting the next couple of pages of the socre, so that when I turn to the next page it appears instantly -- for all PDF files, even those with high-resolution scanned images.
Easy ways to jump to page numbers and rehearsal numbers quickly, while rehearsing.
Ways to add bookmarks to scores for rehearsal numbers
Some way of annotating the score. 
I can think of many features which would make the app even better.


Can any of you recommend iPad apps which work well for you as PDF score readers?
forScore

This is the app I use--it's extremely stable and has a fantastic feature set. Only $5, too. What initially drew me to it was how easy it is to get files into the app--you can download them from a URL, from your Dropbox, upload from iTunes, use a built-in store, take a picture of a physical score with your iPad's camera... There's also a neat feature to share scores with other iPads over Bluetooth--something I've on occasion done with my piano teacher.

I don't even use all of the many features, but it's an app designed specifically for PDF sheet music reading and performance, so here are some I know off the top of my head:


Lots of options for metadata, categorizing, and bookmarking exist.
Annotations are built-in and very easy to use.
It's got a handy "performance mode" that disables extra functions except for the page turns.
It can even make use of a Bluetooth foot switch accessory for page turning.
You can rearrange/duplicate/crop the pages of your PDF so you have a linear set of page turns to deal with cuts, repeats, and codas.
You can bind page turns to an audio track for rehearsal or performance purposes.
Built-in metronome and pitch pipe for rehearsal purposes.