PDF size is usually driven by a small number of things: embedded images, scanned pages, fonts, duplicate resources and the way the PDF was produced. Before compressing a file, it helps to know which of those is actually making it large.
Image-heavy PDFs have the most room to shrink
A PDF created from a set of high-resolution photographs or scans can be much larger than a text-only document. Reducing unnecessary image resolution or compression overhead can produce a meaningful size reduction.
A practical rule is to match image resolution to the destination. A document intended only for on-screen reading does not necessarily need the same image resolution as a high-quality print file.
Text-only PDFs may barely change
If a PDF is already optimized and contains mostly text, there may not be much redundant data to remove. A small reduction is not automatically a sign that compression failed; it may simply mean the original file was already compact.
Avoid quality claims that the file cannot support
There is no universal “compress by 90% with no quality loss” setting. Lossless techniques can remove some structural overhead, but large reductions in image-heavy files often involve changing image encoding or resolution.
After compression, compare both file size and readability. Zoom into small text, inspect images and check that pages still render correctly.
A practical workflow
- Keep the original PDF as your backup.
- Run Compress PDF.
- Compare the new file size with the original.
- Open the compressed PDF on the device where it will actually be used.
- If the result is still too large, consider reducing oversized source images before recreating the PDF.
For image-heavy workflows, Resize Image can help you prepare large images before they are placed into a new document.
Bottom line
The best compression is the amount that solves your delivery problem while keeping the document readable. If a PDF is already optimized, accepting a modest reduction can be better than aggressively degrading its images.