There’s a lot of variability in accuracy of recognition of images of text characters in digital images, in ability to correct for skew and in availability of large dictionaries in the OCR software.
In short, some OCR applications are much more accurate than others in recognizing and converting images of text to searchable PDFs. The more OCR errors, the more likely it becomes that a specific word you want to search for will not be found.
I don’t know of any OCR app running on a mobile device that I would expect to approach the accuracy of OCR by ABBYY OCR software. I’ve tested and used a lot of the OCR applications, both on mobile devices and under OS X (Abbyy FineReader 12.0.4, Abbyy in DEVONthink Pro Office, ReadIRIS Pro, Acrobat Pro, OCRKit, PDFPenPro–and rank ABBYY best overall). I have taken images of book pages from my iPhone 5S using Scanner Pro, and gotten pretty good OCR accuracy after doing additional image processing, then OCR on my Mac using ABBYY.
At this time, none of the OCR apps for mobile devices can approach the recognition accuracy of the better (or even “less better”) OCR apps running under OS X.
There’s also variability in image quality, contrast, and white balance among scanners. Not all scanners are equal. I once ran scans of the same paper copy through four different scanners (all set for 300 dpi) and found a surprising number of errors in scans from one of them, which produced images with poor contrast. My personal favorite is the ScanSnap, which tested best in that comparison. (Yes, there are other good scanners. No, I’m not going to name the others in that batch of four that I tested. I don’t have access to all brands and models of scanners.)
Obtaining error-free OCR is really, really difficult. We humans can recognize and interpret images of text more accurately than any current OCR software. But OCR makes images of text searchable, and that’s very important to me. Generally, I can search PDFs made from clean paper copy, on a good scanner and with OCR by competent OCR software with a very good likelihood that the word I’m searching for was properly recognized and converted, and so will be found.