If you want the pure Arabic Kitab al-Umm in searchable, high-resolution format, the standard is the edition (edited by Dr. Ridwan Muhammad Ridwan).
If you want English access to Imam al-Shafi‘i’s legal legacy: imam shafi kitab alumm english pdf extra quality
For high-quality digital study, researchers often use the to access complete Arabic scans, while English speakers frequently look to academic libraries or specialized publishers like the Aseel Foundation%20Madkhal/The%20Four%20Imams/Abu%20Zahra,%20Muhammad%20-%20The%20Four%20Imams_%20Their%20Lives,%20Works,%20and%20Schools%20of%20Thought.pdf) for scholarly summaries and translations of the Imam's biography and legal methods. The Four Imams - Aseel Foundation If you want the pure Arabic Kitab al-Umm