PDF compression levels control how much image detail is removed from a file to make it smaller. Low compression keeps almost everything and saves a little, medium is the balanced choice for everyday documents, and high gives the smallest file with visible softening in pictures and scans. In Compress PDF you choose the level with a slider and see the new size and a preview before you download.
What a compression level actually changes
A PDF can contain text, fonts, line drawings and images. Text and drawings are already compact. Images are the heavy part, so that is where compression does its work.
Two things are adjusted as the level goes up:
- Resolution. The number of dots that make up each image is reduced. A picture with more dots than a screen or printer can show loses nothing visible at first.
- Image quality. The picture is saved with stronger compression, which merges fine colour differences. At gentle settings this is invisible. At strong settings you see soft edges and blocky patches.
This explains a result that confuses many people. A text-only PDF hardly shrinks at any level, because it has few images to reduce. A scanned or photo-based PDF can shrink a great deal.
Low, medium and high PDF compression levels compared
| Level | Size saving | Effect on appearance | Best for |
|---|---|---|---|
| Low | Modest | No visible change at normal zoom | Printing, portfolios, ID documents, certificates, archives |
| Medium | Good | Slight softening when you zoom in closely | Email, application portals, reports, scanned letters |
| High | Largest | Photos and small scanned text look soft | Drafts, reference copies, quick sharing on slow connections |
One warning about names. Some tools label their options by quality and others by compression, and the two run in opposite directions. Low compression means high quality. High compression means low quality. Read the label carefully, or simply watch the size figures.
Low compression
Choose this when the document will be printed, zoomed into or judged on its looks. A design portfolio, a product catalogue or a scanned degree with a fine seal all deserve the gentle end of the slider. The saving is smaller, but nothing looks different.
Medium compression
This is the sensible default. Most scanned letters, forms, statements and slide decks come out much smaller and still read cleanly on a phone or laptop. If you do not know which level to use, start here.
High compression
Use the strong end only when size is the main goal. It suits a reading copy of a long manual or a draft you are sending for comments. Avoid it for anything with small print in a scan, faint handwriting, signatures or photos of faces.
How to pick a level with the slider
PDFNova does not force you into three fixed buttons. A slider lets you stop anywhere between gentle and strong, which makes it easier to hit a specific size.
- Open the Compress PDF tool and add your file.
- Place the slider around the middle.
- Read the before and after size. Decide whether you need more saving or more quality.
- Check the preview. Zoom in on the hardest detail in the document, such as footnotes, a stamp or a chart.
- Move the slider a little in the direction you need and check again.
- When the size fits and the detail is clear, download the file.
The whole process happens on your device. PDFNova processes files in your browser, so your documents are not uploaded to a server. For a full walk-through, see how to compress a PDF for free.
Which level for which situation
- Sending by email: medium is usually enough. Check the attachment limit your email service shows.
- Uploading to a job or university portal: start at medium, then adjust until the size is just under the limit on the upload page.
- Printing at a shop: low, or no compression at all.
- Sharing in a chat app: medium to high, depending on how much detail the reader needs.
- Storing for the long term: keep the original, and add a medium copy for everyday use if space is tight.
- ID cards, passports and certificates: low. Numbers and photos must stay sharp.
Document type matters as much as the level
The same slider position gives very different results on different files.
Photo PDFs. Files built from camera pictures carry a lot of spare detail, so even a gentle level makes a big difference. See how to make smaller PDFs from photos for ways to keep them light from the start.
Presentations. Slide decks mix text with large background images and screenshots. Medium compression usually works well, but check slides with detailed charts. Size is also affected by how you export, which is covered in how to save a PowerPoint as PDF, with or without notes.
Scanned text. Letters are part of the image, so they soften as the level rises. Judge by the smallest text on the page.
Digital text documents. Exported reports and essays stay sharp at every level, and also save very little. If one of these is too big, look for embedded images or extra pages instead.
Mistakes to avoid
- Compressing a compressed file again. Quality drops each time while the saving gets smaller. Go back to the original and choose a stronger level in one pass.
- Judging by the first page. Check the page with the most detail.
- Using high for everything. A tiny file that nobody can read has no value.
- Throwing away the original. Detail removed by compression cannot be brought back.
If even a strong level does not get you under a limit, compression is the wrong tool for the job. Remove pages you do not need, or divide the document with Split PDF and send it in parts.
Frequently asked questions
What is the best compression level for a PDF?
Medium suits most everyday documents. Go lower for printing, portfolios and identity documents, and higher only for drafts or when a strict size limit leaves no other choice.
Does high compression damage the text in a PDF?
Real, selectable text is not affected. Text inside scanned pages is part of an image, so it becomes softer and can be hard to read at high compression.
What is the difference between lossy and lossless PDF compression?
Lossless methods repack data without changing how anything looks, and give small savings. Lossy methods remove image detail and give large savings. Compression levels mainly control how much lossy reduction is applied to images.
Why does my PDF stay the same size at every level?
It probably has few or no images. Text and fonts are already compact, so there is little for any level to reduce.
Levels are easier to understand once you try them on a real file. Open Compress PDF, move the slider from one end to the other, and watch how the size and the preview change.