Noteful is a PDF annotation app with a strong selection of features that make it a great choice for research and teaching.
Pricing
Noteful requires a small one-time purchase ($5 when when I bought it, circa 2024).
Alternatives
Notability - Notability provides more AI features than Noteful, including handwriting-to-text conversion, speech transcription, and automatic generation of study notes. It comes with a significantly higher price tag as a result. Notability also offers a feature for publishing notes online, which can be useful for sharing lecture notes with students.

After trying many PDF annotation apps for iPad, I finally settled on Noteful. Despite being the cheapest among the contenders, Noteful stands out for being exceptionally well-designed with several features that other apps often lacked:

  • Drawing layers that can be enabled and hidden.
  • Document outlines are automatically populated from PDF outlines, resulting in easy navigation.
  • (Nearly) infinity zooming, so that you can read tiny details and make precise modifications.
  • Customizable pen settings allow you to configure the appearance of your handwriting to your desired level of smoothing and pressure sensitivity.
  • Page tags allow you to tag content on a given page for later reference.
  • Stickers allow you to save and re-use an annotation;

Noteful does have a few downsides:

  • No handwriting recognition, so it can be hard to search for content in handwritten notes.
  • No audio transcriptions.
  • When notes are backed up to a cloud service, such Google Drive, notes are organized by tags instead of by folder.