Scans

A page from a scanner is a photograph

That is where the weight comes from, and also why nothing inside it can be searched.

Why it weighs so much

A scanner records dots, not letters. One page at print resolution can outweigh dozens of pages of ordinary text, and a twenty-page contract becomes an attachment that will not send.

What you gain and what you lose

You gain a file that travels by mail and opens faster. You lose some detail in the images — usually invisible on screen, visible in a close print.

When not to shrink

When the document goes to a printer, to a court, or to an office with legibility requirements. Then send the original another way, or split it into parts.