The GIMP Documentation website is the primary source for all versions, including GIMP 2.10 and GIMP 3.0. While they don't provide a direct "Download PDF" button for the latest versions, you can install the manual locally to use it without an internet connection:

The official GIMP help manual is primarily distributed as an online resource or as a locally installable help system rather than a single, official PDF download. However, you can access older PDF versions, use third-party community-compiled PDFs, or install the manual for offline use within the software itself. Official Documentation & Offline Help

For a more condensed guide, platforms like Babraham Bioinformatics offer specific GIMP tutorial PDFs. Quick Tips for Navigating the Manual GIMP - Documentation