Even if you find a "clean" file, the quality is often terrible. Images are heavily compressed, pages are out of order, double-page spreads are split incorrectly, and the translations are often rushed or riddled with typos.

However, the manga industry does not officially release manga as downloadable PDFs. This is the root of the problem.

: For tech-savvy users, this tool can take raw image files from volumes and compile them into a high-quality PDF specifically formatted for double-sided printing. Free Official Apps (Mobile-Friendly)

The legal landscape has improved dramatically. You can read thousands of chapters for free via ad-supported official apps (Shonen Jump, Manga Plus, Azuki). You can download them offline within those apps.