It appears there was an attempt at a free Android OCR library using the Tesseract OCR engine here. However, the code there seems to be rather old (6 to 7 years), and I’m not sure if anyone tried to update it or build a newer free library. I also don’t know if there’s any documentation for that library.
As for iOS, it appears someone created a wrapper for Tesseract OCR to work on iOS, as explained by this article..
If the above suggestions don’t pan out, and you can’t get a free solution that works for you, you could consider using a professional API like LEADTOOLS. (Disclaimer: I am an employee of this toolkit’s vendor). The toolkit has a powerful OCR engine for both Android and iOS. and there’s a free evaluation edition you can try before you decide whether to purchase or not.
You can also try the LEADTOOLS demo OCR applications from the Google Play Store or the Apple App Store.