What makes a scan look like a scan
A digital PDF has perfectly straight text, a pure white background and selectable letters. A scan is different: the sheet is slightly rotated, the paper has tone and texture, there's a shadow from the scanner lid and the image is a bit soft and grainy.
Let's Scan reproduces exactly that. Each page gets its own slight random skew — as if the sheets went through the scanner one by one.
What happens to the file
PDF pages are rendered in your browser, the effect is applied and a new PDF is assembled from the results. The original file is not modified or sent anywhere. Page sizes are kept, so the document prints at the same size.