Scanned PDF's will not be searchable unless they are put through an OCR engine first - otherwise they're just jpg compressed graphics files (or .bmp depending on your pdf settings) wrapped inside a .pdf file. 'Printed' .pdf's consist of text data, embedded fonts embedded pixel graphics, and embedded vector graphics depending on the source application.

Since text, fonts & vectors are 'small' data compared to pixel graphics, the resulting file will be smaller than a purely pixel graphic of a scanned .pdf - even if they look identical to the eye.

OCR would replace the pixel information with text information, creating searchable text and shrinking your file size. No OCR on scanned documents = no search capability.

-Zeke