PDF pages do not have one universal DPI
A PDF page is measured in physical units and can contain text, vectors, and several raster images at different sizes. Text and vector paths are resolution independent. A raster image has an effective DPI only after its pixel dimensions are compared with the physical size at which it is painted on the page.
The same image object can therefore have different effective DPI values when reused at different display sizes in the same document.
Effective DPI comes from pixels divided by printed size
For one axis, effective DPI is approximately image pixels divided by displayed inches. A 1200-pixel image displayed four inches wide is about 300 DPI horizontally. If it is stretched to eight inches, the same pixels produce about 150 DPI.
OpenFileTools measures supported image placements rather than assigning a guessed DPI to the whole PDF.
Use resolution bands as review signals, not guarantees
For many ordinary print workflows, 300 DPI or higher is a common high-quality target for photographs and fine raster detail, while 150–299 DPI can be usable depending on viewing distance and output method. Values below roughly 150 DPI deserve closer review for print. These are practical review bands, not universal printer acceptance rules.
- Large-format graphics may be viewed from farther away and need less effective DPI.
- Line art may need more resolution than photographic content.
- Vector logos and text should not be judged by raster DPI thresholds.
Know what a browser DPI check cannot measure
Some masks, unusual image encodings, malformed objects, transformations, or protected PDFs may not expose reliable pixel-placement evidence. Unknown measurements stay unknown. For press-critical production, combine the browser report with the printer's own preflight requirements.