Readdle is offering a sale on their apps for the next 48 hours. In particular, you might want to consider PDF Expert, which is only $4.99 right now. PDF Expert is the best way that I have found to interact with “official” PDFs–filling out PDF forms, business documents, etc. There was an update this last week that now allows PDF Expert to merge PDF documents together.
No, you won't want to use PDF Expert to manage your digital sheet music (although, if you have PDFs that you need to edit on your iPad, such as moving pages, making multiple PDFs out of one document, etc., PDF Expert is probably your best bet). It serves a different purpose than PDF music readers.
Additionally, you will want to check out Readdle's Documents, which is always free. Documents is a great workflow solution if your district isn't using Edmodo or Schoology with iPads. For example, if you create a document in Pages, you cannot save it to Google Drive. But you can save a document to Documents, and then move it to Google Drive. You can also be logged into multiple Google Drive accounts (e.g. personal and school).
Readdle has been around for six years, and they have several other apps as well–this is as good of time as any to get some of their apps!