Choose a mode based on what you need to preserve
Start with content-preserving cleanup. It rewrites the PDF without turning pages into pictures, so selectable text and interactive content are retained. A PDF that is already well optimized may not become smaller.
The image-page modes render pages at 150 DPI (balanced), 96 DPI (smaller), or 200 DPI (higher quality). These modes can shrink scan-heavy documents, but they flatten the pages: selectable text, links, forms, bookmarks, and accessibility information are lost. Any rewrite may invalidate a digital signature. Keep the original file.
What a good result looks like
Compare the original and output sizes shown after processing. A smaller number is useful only if the pages remain readable at normal zoom and when printed at the intended size. Check fine print, diagrams, colors, and every page before sharing.
For example, if a scan is 8 MB and image mode returns 3 MB, the saved percentage is 62.5%. That example is illustrative, not a promised result. If the tool cannot make the file smaller, it offers the unchanged original instead of claiming a saving.
Troubleshooting large or difficult PDFs
Photo scans tend to benefit most from downsampling. Text-heavy digital PDFs often benefit little because their content is already compact. Grayscale can reduce some files but changes colored marks and highlights.
Image mode is limited to 150 pages and large pages may be rendered at a capped resolution to protect browser memory. For an oversized document, split it into smaller parts or process it on a device with more memory.