PREP allows you tag scanned PDFs and also edit the OCR text content in a Scanned PDF. Ensure, the pdf document is scanned properly and lines are not skewed for a better result. 

Follow these steps to tag a scanned PDF:

Step 1 : Fix the tag types, reading order for all the pages

Step 2 : Go to OCR mode, and review the OCR content for pages and make edits wherever necessary

Step 3 : Come out of OCR mode and Export the PDF document

Tips :

1. Finish tagging the document, only then try to edit OCR content. Also, after editing OCR content, don't change the tag structure as it might cause issues.

2. You need to wait a little longer while uploading scanned documents.

3. Ensure PDF is scanned properly before uploading to PREP. Especially, verify skewed lines in the OCR mode.