Scannable outputs a JPG or PNG when it outputs an 'image'. It dynamically switches between the two options (PNG or JPG) based on the image content.
Scannable uses a custom lossy PNG compression scheme and color palette minimizer that results is better looking (sharper) with lower file sizes.
In practice, this means your typical black-print-on-white paper document, perhaps with a colored title, taken on iPhone 6, will be in the 200-400k size, with no fuzzy JPG compression around the text letters. A JPG with similar quality is typically 600-800k range and will have blurrier looking text.
If the results are larger in filesize than they would be for JPG, or unable to authentically reproduce the colors in the document, Scannable uses the JPG format.
The number of pixels remains unchanged between JPG and PNG.
The reason for doing this is
a) smaller filesizes
sharper looking documents
c) faster uploads
This also applies to PDF, where Scannable embeds a PNG instead of JPG on each page.
Having said all this, we'd love to know if you need a JPG file specifically for a technical reason (For example another app or web service that can only accepts JPGs) or weren't happy with the technical quality of the images? Could you describe your use case a little?
Search within Scanned PDFs is a Premium only feature. Scannable makes PDFs for multiple-page scans, by default. You can change this in settings. Please refer to https://evernote.com/contact/support/kb/#!/article/23258452
And some more on the PDF search functionality itself: https://evernote.com/contact/support/kb/#!/article/23169032