Practical guide 3 min read
How to make a PDF smaller without losing quality
Shrink the photos and scans inside a PDF and leave its text sharp: which compression level to pick, what changes, and what to do when a PDF won't shrink.
Why PDFs get big
Most of a large PDF's size is pictures: scanned pages, phone photos of documents, or the photos in a brochure. A Letter page scanned at 300 DPI is 2,550 × 3,300 pixels, about 8.4 million of them, which is far more than a screen shows or a printer needs. A page of text, by comparison, takes a few kilobytes plus the fonts it uses.
So the reliable way to make a PDF smaller is to shrink its images and leave its text alone. Compress PDF does exactly that, in your browser, without an account.
Compress a PDF, step by step
- Open Compress PDF and choose or drop your file.
- Leave Compress by on Level and pick Recommended.
- Select Compress PDF. The result shows the old size, the new size and how much smaller it is, and the file downloads.
- Open the new file and zoom in on the smallest print you need to read before you send it.
Your original file isn't changed, so you can always try again at another level.
Which level to choose
| Level | Images kept at up to | Best for |
|---|---|---|
| Light | 200 DPI | Documents you'll print, photos you want crisp |
| Recommended | 150 DPI | Email, most upload forms, reading on screen |
| Strong | 100 DPI | The smallest file that still has real text |
The DPI is measured against the page. On a Letter page, which is 11 inches tall, Recommended lets an image be up to 11 × 150 = 1,650 pixels on its long side. Halving the DPI keeps a quarter of the pixels: the 300 DPI scan above becomes 1,275 × 1,650 at 150 DPI, about 2.1 million pixels.
Images already smaller than the limit keep their size and are only saved again if that makes them smaller.
What changes and what doesn't
- Text stays text. It's still selectable, searchable and sharp at any zoom.
- Vector drawings are untouched. Lines, shapes, charts and logos drawn in the PDF itself don't change.
- Photos and scans are resized and saved as JPEG at the level's DPI and quality.
- Lossless images (screenshots, or charts saved as pictures) are only converted when that at least halves their size, because JPEG softens sharp edges.
- Kept as they were: transparency, form fields, links and bookmarks.
- Removed, because nothing shows them: hidden editing data that Illustrator and Photoshop store in PDFs, page thumbnails, and XMP metadata.
A worked example
A three-page letter scanned in greyscale at 300 DPI came to 6.5 MB. On Recommended it came out at 508 KB, 92% smaller, and the pages look the same at normal zoom. A 4.0 MB PDF holding one 300 DPI photo went to 158 KB.
Your numbers depend on what's on the pages: busy photos keep more detail than a plain white page with a few lines of type.
When a PDF barely shrinks
If the tool says there are no photos or scans to shrink, the PDF's size is fonts and text, and it's already close to its smallest. To go further:
- Remove what you don't need with Delete PDF Pages.
- Send it in parts with Split PDF.
- Switch to Target size. As a last resort it can save pages as images, which makes the file smaller but stops the text being selectable.
Where your file goes
The PDF is read, compressed and saved in your browser. It isn't uploaded to LabAnvil or anyone else. The page still loads website assets, and analytics may apply as explained in the privacy policy.