There are some PDF's, specifically with brokerage statements from Fidelity, that stay uniformly smooth when zooming in.  These are likely created from some form of a vector image.  Scan cannot reliably read these files and the data extracted in some areas is good  but others is not readable.

To resolve this issue, do the following:

1. Open the PDF file.
2. Click PDFlyer.
3. Click Export to multi-page tiff.
4. Run the file through Scan.