The PostgreSQL documents are written in SGML, which is similar to, but not the same as, XML. These files are then processed to generate HTML files, PDFs, and so on. This ensures that all the formats have exactly the same content. Then you can choose the format you prefer, and you can even compile it in other formats, such as EPUB, INFO, and so on.
Moreover, the PostgreSQL manual is actually a subset of the PostgreSQL source code, so it evolves together with the software. It is written by the same people who make PostgreSQL. Even more reasons to read it!