Another idea: OCR, which goes hand-in-hand with search indexing. There's obviously other ways to do this with 3rd party applications, but a built-in, streamlined process (and without need for the command-line) would be great. Again, Zotero has a plugin for this, that is built off of the free Tesseract engine.
https://github.com/UB-Mannheim/zotero-ocr