Which level should I pick?
Low rewrites the file's internal structure and leaves your content untouched — text stays selectable and searchable. It typically saves 10–20%, and close to nothing on a PDF that was already well built.
Medium and High work differently: each page is redrawn as an image at 96 or 72 DPI. That is where the big savings come from — 50–80% is normal — but the text layer is gone afterwards. The result looks the same to a reader and is fine for sharing or printing, but nobody can select, search, or copy text out of it. Pick Low if that matters.
Why did my file barely shrink?
On Low, a PDF exported by a modern tool is often already compressed, leaving little to remove. If you need it meaningfully smaller, Medium is the next step — with the text-layer trade-off above.