What's the best iOS app for editing and annotating PDF files on an iPhone?

Flexi PDF is a mobile application tailored for handling PDF documents directly on iOS devices, particularly for editing and annotation tasks. It stands out for its user-friendly interface that simplifies common PDF manipulations, such as adding or modifying text, inserting comments, and highlighting sections. Specific features include a robust text editor for precise adjustments, a range of annotation tools like sticky notes and drawing pens, and digital signature capabilities for signing contracts or forms on the go. In real-world scenarios, business professionals can quickly review and mark up reports during commutes, while students might annotate lecture notes without needing a computer. Reviews indicate that users appreciate the app's responsiveness and the ability to save edited files in various formats. However, some feedback points to minor limitations, such as occasional delays when handling large PDFs or the need for in-app purchases to unlock advanced editing options. Despite these quirks, Flexi PDF provides a reliable solution for mobile PDF editing, balancing functionality with accessibility for everyday use.