GitLab

* fix confusing PIL instructions

As stated in the documentation
[here](https://pillow.readthedocs.io/en/stable/handbook/image-file-formats.html?highlight=pdf#write-only-formats

),
PIL can only write PDF's, not read them. Remove references to reading
PDF's via PIL from this page to avoid confusion.

* mention PDF in doc examples using PIL
Co-authored-by: NielsRogge <48327001+NielsRogge@users.noreply.github.com>

* Be explicit: PDFs must be converted to images

* fix formatting
Co-authored-by: NielsRogge <48327001+NielsRogge@users.noreply.github.com>