Classify pdf files - image approach vs. text approach
Both methods will be beneficial for different cases.

If you think that dependencies in the text are more discriminative of the classes, then NLP apporach. Image approach in this case will need to be super complex to catch this kind of information.

On the other hand layout and position could be very informative and this it could happen is not encoded in text and only image approach can catch this information.

Conclusion. Maybe think about hard-encoding layout and position features and pass them along to NLP model.