Understanding Analysis is published by Springer as part of their Undergraduate Texts in Mathematics series. Many universities provide free institutional access to the full PDF version via SpringerLink. If you are enrolled in a college or university, log in through your campus library portal to download the official digital version legally.
Many free PDFs floating around are of the (2001). The second edition (2015) includes: understanding analysis stephen abbott pdf
Some of the key topics covered in "Understanding Analysis" include: Understanding Analysis is published by Springer as part