New user - OCR / search seems unreliable

I seem to be able to drag and drop certain pdfs into a DT folder and be able to search them immediately. Others need to be converted before being ocr’d and searchable, while still others cannot be ocr’d/searched regardless of how I import them. So what am i missing? And what/where are the “rules”? Thanks.

The rule (if there is one) is the resolution of the images in the PDF. In general OCR doesn’t work very well with resolutions less than 300dpi.