First identify what makes the PDF large
Large scans and photographs often account for much of a PDF’s size. Other documents may contain vector drawings, embedded fonts, or already-compressed images. The right approach depends on what the pages contain.
Keep an untouched copy before trying a new compression setting. If the document is already optimized, a second pass may not help.
Image compression changes the document
Rasterization turns each page into an image. This can reduce the size of some scanned documents, but it can also make text less sharp and removes selectable text, links, form fields, and vector detail. The output may be larger for some source PDFs.
An image-quality slider balances visual detail against image size; it cannot guarantee a particular file size or preserve the original document features.
Compare before replacing your original
Open the output and compare small text, diagrams, signatures, and page edges at the zoom level where the PDF will be used. Check that every page is present and legible, then compare file size.
If searchable text or forms matter, do not use a rasterized copy as a replacement. Use the Compress PDF tool only when an image-based output fits your needs.