remove references to PDF reading via PIL
As stated in the PIL documentation
[here](https://pillow.readthedocs.io/en/stable/handbook/image-file-formats.html?highlight=pdf#write-only-formats),
it can only write PDF's, not read them. Remove references to reading
PDF's via PIL from this page to avoid confusion.

## Before submitting
- [X] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).

The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_15293). All of your documentation changes will be reflected on that endpoint.<|||||>This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

Please note that issues that do not follow the [contributing guidelines](https://github.com/huggingface/transformers/blob/master/CONTRIBUTING.md) are likely to be ignored.<|||||>Not stale, just needs a quick review and a merge.<|||||>No problem, Niels. I think the wording you suggested is still a bit confusing, but since you want to mention PDF I'll send back a counter-edit :) <|||||>@NielsRogge I have built on your comment, making it explicit that PDFs have to be converted to images first.<|||||>This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

Please note that issues that do not follow the [contributing guidelines](https://github.com/huggingface/transformers/blob/master/CONTRIBUTING.md) are likely to be ignored.<|||||>Looks like I need to come back and style the file I changed.<|||||>_The documentation is not available anymore as the PR was closed or merged._<|||||>@NielsRogge I fixed the line lengths for the lines that we actually changed. The line length test is still failing, but it's because there are other lines in the file that are longer than 119 characters.