How to reduce PDF file size without losing quality (what actually makes PDFs big)
A PDF is big for one of four reasons, and each has a different fix. How to find out which one you have, what 'light', 'medium' and 'strong' compression really do, when text stays selectable, and how to hit an exact size limit for a portal or e-mail.
· 3 min read · 1.4K views · by the Cortex Digital Hub team
Shrink it in your browser, nothing uploaded. Light, medium, strong, or straight to a target size in KB or MB. Free.
Compress a PDF now → Opens Compress PDF · runs in your browser
"Compress PDF" is one of the most-searched phrases on the internet, and most of the answers skip the only question that matters: what is making the file big? Four things can, and the right fix depends on which one it is.
The four reasons a PDF is large
1. Photos embedded at full resolution. A 12-megapixel phone photo is 3–5 MB and keeps that weight inside the PDF, even when it is displayed at the size of a postage stamp. Three photos per page, five pages: 50 MB. This is the usual cause for forms with photos attached and for brochures.
2. Scans at high dpi. A document scanned at 600 dpi is four times the pixels of a 300-dpi scan and sixteen times a 150-dpi scan. For reading on a screen, 150 dpi is plenty; for printing, 200–300.
3. Embedded fonts, many of them. A design-tool export can embed every weight of a font family on every page. It is rarely more than a few MB, but it is why a text-only PDF from InDesign can be 8 MB.
4. Hidden extras. Previous versions kept inside the file, form data, embedded files, thumbnails, metadata. Common in PDFs edited many times in Acrobat.
How to tell which one you have
Divide the file size by the page count. Under 100 KB per page: fonts or extras, and a light re-save fixes it. 300 KB to 1 MB per page: images at reasonable resolution, and medium compression gets most of it back. Several MB per page: full-resolution photos or 600-dpi scans, and only re-encoding the images helps.
What the compression modes really do
Most tools, including the Cortex PDF compressor, offer three levels, and they are different operations, not three strengths of the same thing:
| Mode | What happens | Text stays selectable? | Typical result |
|---|---|---|---|
| Light | Re-saves the file: drops extras, re-encodes images that are oversized for their display size | Yes | 10–60% smaller |
| Medium | Renders each page to an image at 150 dpi, JPG | No (becomes a picture) | 60–85% smaller on scans |
| Strong | Same at 100 dpi, higher JPG compression | No | 80–95% smaller; fine for WhatsApp |
| To a size | Tries light, then medium, then stronger until the file fits the KB or MB you typed | Depends on how far it had to go | Exactly what the portal wants |
"Without losing quality" therefore means: light mode when the PDF has real text, because the text is untouched and only the images are brought down to the resolution they are shown at. For scans there is no text layer to lose; "quality" means readable at the resolution you choose, and 150 dpi reads fine on any screen.
Hitting an exact limit
Portals and e-mail have limits: 100 KB, 200 KB, 500 KB, 1 MB, 2 MB for most Indian government and job portals; 25 MB for Gmail; 20 MB for Outlook; 100 MB for WhatsApp documents. Type the number into "To a size" and let the tool find the gentlest setting that fits, rather than guessing a mode. If you need the common sizes, there are pre-set pages for 100 KB, 200 KB, 500 KB, 1 MB and 2 MB.
Greyscale, and when to use it
A colour scan of a black-and-white document carries three channels of data for no benefit. Greyscale cuts the size by a third to a half at the same dpi and the text looks identical. Use it for forms, statements and certificates; not for anything with a photo or a colour stamp that must stay red.
Before you compress: fix the source
- Photos going into a form: resize them to 1000–1500 px first with the image resizer, then make the PDF. A 300 KB photo in a PDF is a 300 KB page.
- Scanning: 150 dpi greyscale for text, 200 dpi for anything with small print. Not 600.
- Exporting from Word or Google Docs: choose "minimum size" or "web" quality at export; it re-encodes images on the way out.
Privacy
Most online PDF compressors upload the file to their server and keep it for a while. For a passport scan, a bank statement or a signed contract that is a bad trade for a smaller file. The Cortex compressor runs entirely in the browser: the PDF never leaves your device, which is also why it works offline once the page has loaded.
Frequently asked questions
Why is my 3-page PDF 15 MB?
Almost always images: a phone photo embedded at full resolution on each page, or a scan at 600 dpi. Text alone weighs almost nothing; a 100-page text-only PDF is usually under 1 MB.
Will compressing a PDF make the text blurry?
Not if the PDF has real text. Light compression keeps the text layer as it is and only re-encodes images. Medium and strong modes convert pages to images at 150 or 100 dpi, which keeps them readable but no longer selectable.
What is the smallest a scanned PDF can go?
A scanned A4 page at 100 dpi in greyscale JPG is about 60–120 KB; at 150 dpi in colour 150–300 KB. A 10-page scan therefore bottoms out around 1 MB unless you go to 72 dpi, which is barely readable.
Is it safe to compress a PDF online?
Only if the tool does the work in your browser. Most online compressors upload the file to a server. A browser-based tool never sends the file anywhere, which matters for ID proofs, bank statements and contracts.