Mark what matters
Use a translucent highlight for a definition or passage, and a text note for your own explanation. Unlike a screenshot, the annotated copy retains the original PDF pages.
Work through a problem
Draw an arrow to a formula, circle a result or sketch a short explanation. Use Undo and selection to correct a mark before saving.
Keep the result
Download the PDF after each study session. The editor keeps only the last document on this device, so opening another file replaces it; the downloaded copy is your record of the session.