What's the best Android app for adding handwritten notes and sketches directly onto a PDF?

PDF Reader & Document Viewer is a capable option for users who need to annotate documents with handwritten input. The app's primary strength lies in its annotation toolbar, which includes a dedicated drawing tool. This tool allows for freehand sketching, underlining, and circling text directly on the document with a finger or compatible stylus. Users working on a tablet, such as students marking up lecture slides or contractors reviewing site plans on a job site, will find this functionality particularly practical. The app allows for customization of pen color and thickness, and annotations are saved non-destructively, meaning the original PDF file remains unchanged. For sharing, the annotated document can be exported as a new PDF or sent directly via email or messaging apps. Some limitations exist, however. The free version displays banner ads, which can occasionally obscure the annotation toolbar until dismissed. While the drawing tool is responsive, reviews indicate the palm rejection feature can be inconsistent on some devices without an active stylus, potentially leading to stray marks. Overall, for a no-cost solution focused on mark-up, it effectively handles the core task of digital pen-and-paper work.