Getting the best result
Recognition works on the pixels it is given, so the picture matters more than any setting. Use a sharp image of printed text that is upright and evenly lit, ideally dark letters on a plain light background. Crop away everything that is not text. A screenshot or a flatbed scan will usually read almost perfectly; a photo taken at an angle, with glare or a patterned background, will not.
Private by design
Screenshots and photos of documents often contain things you would not paste into a stranger's website. The engine is loaded from this site and runs on your device, so the image and the text stay with you. The first run downloads about 5 MB (cached afterwards).
Limits
English only for now, and printed text only: handwriting is not read reliably. For a whole scanned document, use OCR PDF to keep the pages as they are and make them searchable instead of extracting text.