I have had good luck using Notability for iOS:
Easy to import PDFs, easy to highlight, easy to annotate with handwriting or typing, and easy to export back to PDF. Fairly responsive on my old first generation iPad.
The only drawbacks I've found are:
1. Documents with handwriting or free form drawing get larger r than I would expect (~ 1 MB per page for a page of Calculus homework or doodles)
2. Doesn't support ssh or git for uploading/downloading documents