The Snap and Read Universal iOS app cannot directly access a PDF. For PDF's you have loaded into iBooks or another app on your iOS device.
Open up the PDF in iBooks (or another app).
Take a screen shot of each page.
Open the Snap&Read iOS app.
Import one of the images you took from your photos. (Article to upload image to Snap&Read Universal iOS is linked below. )
Use the Snap&Read OCR tool to read the image.
Link to uploading an image to Snap&Read iOS: