PDF DPI checker: measure effective image PPI

What “PDF DPI” actually means

People commonly say PDF DPI, but the useful source-image measurement is effective pixels per inch (PPI). Printer DPI describes output dots, which is different. Text and vector artwork are described mathematically and do not have a fixed raster PPI. A scan may be one large image; a designed PDF can contain many images with different resolutions.

For example, a 600-pixel-wide image placed 2 inches wide is 300 PPI. Place that same PDF image object 4 inches wide and it becomes 150 PPI. Averaging those placements into a single “PDF DPI” would hide the weaker one.

How this checker measures a PDF

1. Parse each page locally.

   PDF.js 3.9.179 reads the page operator list in browser memory. The document is not sent to a conversion endpoint.

2. Follow the graphics-state transforms.

   The checker tracks save, restore, and transformation operations that determine the physical width and height of each supported raster image placement.

3. Measure every placement.

   The image pixel dimensions are divided by the transformed width and height in inches. Reuse of the same image at different sizes creates separate rows and separate effective-PPI values.

4. Compare with your target.

   The lower of horizontal and vertical PPI is compared with the adjustable target. This is a diagnostic comparison, not a claim that the whole PDF will print correctly.

Reproducible PDF image-resolution fixture

Our four-page CC BY 4.0 fixture contains four known raster placements plus a vector-only control page. Page 1 and page 2 deliberately reuse the same 600 × 300 pixel image at different physical sizes. The expected values are not inferred from this browser checker: the included evidence comes from Poppler’s independent pdfimages -list command.

Download the four-page fixture Expected-results manifest · Poppler evidence · PDF metadata evidence

Source rasters: 600 × 300 PNG · 800 × 600 PNG · 300 × 300 PNG. The manifest records SHA-256 checksums for all published fixture files.

Is 300 PPI required for printing?

No single PPI threshold proves that a PDF is print-ready. Adobe’s PDF settings documentation describes downsampling choices and says image resolution should be appropriate for the intended output device. Acrobat Preflight can inspect image resolution as one of many production checks. A commercial printer may request 300 PPI for continuous-tone images, while line art, large-format work, screens, or distant viewing can call for different targets. Ask the receiving printer for its specification.

Useful primary references: Adobe PDF conversion settings, Adobe Acrobat Preflight, and the published PDF 1.7 specification.

Limits and interpretation

This checker measures common PDF image XObject and inline-image paint operations exposed by PDF.js. Unusual repeated/grouped operators are disclosed in the report instead of silently treated as complete coverage. Clipping, transparency masks, halftone settings, resampling later in a print workflow, printer screening, sharpening, and visual subject matter can affect output quality but are not judged here. A page with no reported raster placement may be vector-only, text-only, or use an unsupported construction.

Image IDs are PDF.js identifiers, not guaranteed original PDF object numbers. Pixel totals are intentionally not averaged into a document-wide DPI. Passwords and selected files remain in memory; CSV and JSON exports are created locally.

Frequently asked questions

Related PDF and print diagnostics