Small e-ink screens (e.g. 6 inch) are only suitable for paperback-sized, text-only books (fiction or non-fiction). Small e-ink screens are simply not suitable for reading PDFs as you have discovered. Even ePub or Kindle programming books are often poorly converted from the print edition of a book.

A general rule of thumb when deciding whether to purchase an e-book: If the physical book is larger than a small paperback size, then it is unlikely to be suitable for an e-ink screen (unless the book is text-only).

Small e-ink readers are not suitable for any books with complex layouts, layouts designed for larger books, or books with colour graphics: charts, diagrams, photos, etc. Amazon, in particular, encourage publishers to convert as many books as possible to Kindle format regardless of whether those books are suitable for e-ink screens.