remove references to PDF reading via PIL (#15293)
remove references to PDF reading via PIL (#15293)

* 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 <e45a0ab1d7a9729bf40010a5d71678f7e86d039d@users.noreply.github.com>

* Be explicit: PDFs must be converted to images

* fix formatting

Co-authored-by: NielsRogge <e45a0ab1d7a9729bf40010a5d71678f7e86d039d@users.noreply.github.com>
huggingface/transformers,huggingface/transformers,huggingface/transformers,huggingface/transformers