Open the PDF and zoom to 200%: if edges fall apart immediately, it was already downsampled and a second pass will only hurt.
On screen only: 150dpi. Must print: 200-300dpi. Plain text document: greyscale or binarise.
Compress, then zoom into a dense paragraph. If the smallest type still reads, the setting is safe for the whole file.
A 300dpi colour A4 scan is roughly 2500x3500 pixels. Uncompressed that is about 25 MB; inside the PDF it lands at 1-3 MB per page. Size scales almost linearly with page count and quadratically with resolution — nothing else matters much.
Downsampling 300 to 150dpi cuts size by about 70% and stays readable on screen and in print. Greyscale saves another 40% but turns red seals grey. Binarisation can reach a tenth of the size and suits plain-text documents, but destroys photos and pale stamps.
Filed as evidence, queued for OCR, or inspected at high zoom — drawings, medical images, handwriting — resolution is the point. Split the file or send a link instead of trading detail for megabytes.
Usually yes at 150dpi greyscale. A 100-page colour scan at 300dpi will not, no matter the tool.
It makes it harder. Below 200dpi recognition rates drop, so keep an uncompressed copy if you still need to extract text.
Greyscale was selected, or the tool converts by default. Red stamps and highlights lose their colour distinction.
No. Compression rewrites the image stream, not the page box.