Pixels and physical size have to be considered together
A 2400×3000-pixel image can be excellent on a small printed page and mediocre if stretched across a poster. Effective resolution is roughly the number of pixels divided by the number of inches those pixels cover. A PDF can place the same image at many different physical sizes, so the useful DPI is a property of that placement, not merely a label in the image file.
A simple example
If an image is 1800 pixels wide and is placed 6 inches wide on the page, its effective horizontal resolution is about 300 pixels per inch. If the same image is stretched to 12 inches, that falls to about 150 pixels per inch. The underlying pixels did not change; the placement did.
Why scanned PDFs become huge
A scanner may create a full-page colour image for every page. At high pixel dimensions, a 50-page document can contain hundreds of millions of pixels. Compression helps, but the dominant cost is still the image data. That is why image-heavy scans often shrink far more than born-digital text PDFs when downsampled.
JPEG quality and resolution are different controls
Downsampling removes pixels. JPEG compression keeps the pixel dimensions but represents them more approximately. Both can reduce size, but they create different artefacts. Heavy JPEG compression can produce blockiness or ringing around text; aggressive downsampling can make small type unreadable even when the JPEG quality setting is high.
| Content | What usually matters most |
|---|---|
| Photographs | Reasonable JPEG quality and enough effective resolution for the target size |
| Scanned text | Legibility of small strokes; grayscale can often be more efficient than colour |
| Line art/diagrams | Sharp edges; lossless or vector content may be preferable |
| Born-digital text | Do not rasterise just to chase a smaller file unless you accept losing text structure |
Why NoblePDF offers separate compression modes
The default Preserve text & vectors mode performs structural optimisation rather than converting every page to a picture. The stronger image-based modes are explicitly labelled because they trade document structure for image compression. That distinction matters: a smaller PDF is not automatically a better PDF if search, copy or accessibility is lost.
How to choose a sensible target
- For email/viewing, inspect small text at 100–150% zoom after compression.
- For printing, test one representative page on the intended printer rather than relying only on screen zoom.
- For archival or legal records, prioritise fidelity and document structure over maximum size reduction.
- For scans, crop unnecessary margins before aggressive image downsampling if the workflow permits.
Worked example: why pixel dimensions alone are not DPI
Suppose an image is 2,400 pixels wide. If it is placed 8 inches wide on a PDF page, its effective resolution is 300 pixels per inch. Place the exact same image 12 inches wide and the effective resolution falls to 200 PPI. No pixels changed - only the physical placement did.
| Image width | Placed width | Effective PPI | Typical consequence |
|---|---|---|---|
| 2,400 px | 8 in | 300 PPI | Usually ample for print-sized text/images |
| 2,400 px | 12 in | 200 PPI | Lower print detail at the larger physical size |
| 1,200 px | 8 in | 150 PPI | May be acceptable for screen viewing, visibly softer in print |
This is why NoblePDF separates image-flattening quality choices from structural compression. Structural compression can reduce PDF overhead without changing the pixel count of an image at all; image-based compression deliberately changes page pixels and therefore needs a resolution/quality trade-off.