I have a big scanned pdf that needs to be smaller for upload but I don’t understand what compression actually changes inside the file. Is there a simple way to explain it?
I ran into this with scanned contracts: most of the PDF size comes from the page images, not the text. Compression either removes duplicate data without changing anything, or lowers image resolution and quality to save more space, so try a moderate setting first and check that small print is still readable.
Think of it like shrinking photos. Scanned pdfs are basically pictures, so compressing lowers the picture quality a bit (fewer dots per inch) so the file takes up less space. Text stays readable, images get a little softer. What are you using to upload it, a website with a size limit?
A scanned PDF is basically just a container holding pictures of pages. When you compress it, it mainly lowers the image resolution and color quality to reduce pixel data, and strips out unused background metadata. The visual clarity might drop a bit, but the file size shrinks significantly.
Simple version: a scanned PDF is really just photos wrapped in a file, so compressing mostly means squashing those photos down. You get a lower resolution and heavier JPEG compression, which is why small files look softer or smudgy. Pages that are real text shrink differently, since they’re fonts and lines rather than pictures. Keep a backup of the original before you try anything.
Scanned PDFs are mostly page images, so compression mainly shrinks those pictures by dropping some quality.
I use PDFcap, just open the file and pick compress.
Tip: go with a medium setting so you can still read it.
Avoid the strongest compression or the pages get too blurry.
Not sure if it’s the same on Mac.
For a scanned PDF, most of the size is usually the page images, not the text. Compression reduces image detail by lowering resolution, using more efficient image encoding, or changing color pages to grayscale or black and white.
OCR can add a searchable text layer, but it usually does not shrink the scan much by itself. I normally keep the original, then test one compressed copy and zoom in on small print to make sure it still reads well.