The 21st edition (2018) can be found used for $20–40. While it lacks COVID-19 data, 95% of core internal medicine remains valid. It’s an excellent budget starter.
If you really want a portable study guide, the official Harrison’s Self-Assessment and Board Review (often available legally as a PDF through your library) contains condensed versions of key chapters. The 21st edition (2018) can be found used for $20–40
If you prefer a PDF but want to stay legal, consider buying a new physical copy of the book. The 22nd Edition (International or US version) often comes with an access code for the eBook version. This gives you the portability of a PDF (via the app/reader) with the reliability of a paid product. The 21st edition (2018) can be found used for $20–40