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A PDF has no DPI — its images do
Ask what DPI a PDF is and there is no single answer. A PDF describes a page: text and vector artwork, which have no resolution and print sharp at any size, and images, each with its own pixel count, each placed on the page at its own size. What a print shop checks is the effective resolution of every image: its pixels divided by the inches it covers on paper.
The same picture can pass and fail in one file. 1200 pixels placed 4 inches wide is 300 DPI; the same 1200 pixels stretched across 8 inches is 150. That is why this tool measures each placement separately, and why an image used twice at two sizes appears twice in the list.
What the report shows
- Every image placement, page by page, numbered on a preview of the page, so you can see which picture a row is about.
- Pixels, printed size and effective DPI for each one. Where the horizontal and vertical values differ, the lower one is shown, because that is the one that decides how the image prints.
- A verdict against your target: enough, under the target, or under half of it. 300 DPI is the usual target for photographs; posters seen from a distance can do with 150; line art sometimes needs 600. Pick what your printer asked for.
- The lowest value in the document and its page — the one to fix first.
Why exporting “at 300 DPI” does not fix a low image
PDF export settings that mention 300 DPI set a ceiling: images above it are downsampled to save space, images below it are left as they are. A small logo placed large is still low after such an export. The only real fixes are a larger source image, or placing the image smaller. Changing the DPI tag inside the image file does nothing either — once an image sits in a PDF, only its pixels and its placed size count.
Scans and one-colour images
A scanned document is usually one image per page, so its effective DPI is roughly the resolution it was scanned at: a page scanned at 300 DPI and kept at its original size reports about 300. Black-and-white scans are often stored as one-colour images; they are listed too, and marked as such.
Nothing is uploaded
The PDF is opened by pdf.js inside this tab and the page previews are drawn locally. There is no endpoint that accepts files; close the tab and nothing remains. For a single photo rather than a PDF, check the DPI of an image; to work out what a print needs, use the DPI calculator.
How to check the DPI of a PDF
Step 1: Drop the PDF in
Drag it onto the box above or click to choose it. The pages are read in your browser, with a progress bar for long documents.
Step 2: Choose the target resolution
300 DPI for photographs in print is the usual choice; switch to 150 or 600 if your print shop asked for that.
Step 3: Read the report
Each page with images shows a preview with numbered boxes and a row per image: pixels, printed size, DPI and a verdict. Tick “show only images under the target” to shorten a long list.
Step 4: Fix the low ones at the source
Replace a low image with a larger original in the program the PDF came from, or place it smaller, then export again and check once more.
Frequently Asked Questions
What DPI should a PDF have for printing?
For photographs, 300 DPI at the size they are printed is the common requirement. Line art and small text in images can need 600; large formats seen from a distance often accept 150. The print shop’s own specification is the one that counts.
Why does the same image appear twice?
Because it is placed twice. Each placement has its own printed size and therefore its own effective DPI — a logo can be fine in the header and too low as a full-page background.
Can this tool raise the DPI of my PDF?
No, and no tool can do it honestly. Effective DPI is pixels divided by printed size; the missing pixels have to come from a better original. Upscaling invents pixels, which softens exactly the edges a print shows.
Does it check fonts, colours or bleed?
No. It measures image resolution only. Font embedding, colour spaces and bleed are separate preflight checks.
Why are some images not listed?
Images smaller than 4 pixels on both sides are skipped (producers use them for lines and dots), annotations such as stamps and form fields are not part of the printed artwork, and documents longer than 400 pages are checked up to page 400.
Is my PDF uploaded?
No. The file is read by pdf.js in your browser; nothing about it — not its name, not its pages — is sent anywhere.
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