PressLint production guide
How to check the DPI of a PDF (3 methods, no Adobe Acrobat needed)
Checking the DPI of a PDF means measuring how many pixels cover one printed inch inside the file. A PDF does not have one single DPI value: every placed image has its own effective resolution, and the same source image can render at 300 DPI on one page and 72 DPI on another if the PDF scales it up. This guide shows three ways to check PDF DPI on macOS, from the quickest (Preview) to the most complete (a dedicated preflight app that audits every image on every page).
What DPI means inside a PDF
DPI (dots per inch) measures how many pixels fit into one physical inch of printed output. A 3000-pixel-wide image placed at 10 inches wide has an effective 300 DPI; the same image placed at 30 inches wide drops to 100 DPI. Print shops typically ask for 300 DPI for color photos and 600 DPI or higher for line art, but the acceptable threshold depends on the press, the paper and the viewing distance. A PDF stores pixel dimensions and a placement matrix per image, not a single document-wide DPI number, so any honest check must inspect each image resource on each page.
Method 1: Check PDF DPI with macOS Preview (quick)
Open the PDF in Preview, select Tools > Show Inspector, then click the Thumbnails tab and select a page. The Inspector shows page size in points (1 point = 1/72 inch). To estimate an image's DPI, use Tools > Rectangular Selection to select the image, note its pixel size from the Inspector, and divide by its physical size in inches. This method is fast but only checks one image at a time and does not account for images the page scales, rotates or crops. It works for a quick sanity check before sending a file to print, not for a full audit.
Method 2: Check PDF DPI with a free command-line tool
Install ImageMagick with Homebrew (`brew install imagemagick`) and run `identify -format '%x x %y\n' input.pdf` to print the horizontal and vertical density reported by the file. For a page-level audit, use `pdfimages -list input.pdf` (from Poppler, `brew install poppler`) to list every image resource with its pixel dimensions and the page it appears on. Divide each image's pixel width by its placed width in inches to get the effective DPI. This catches scaled and stretched images that Preview alone misses, but it still requires manual math and does not produce a shareable report.
Method 3: Check every image DPI with a dedicated preflight app
A preflight tool like PressLint reads the PDF, locates every raster image on every page, reads the page transformation matrix and calculates the effective DPI for each image automatically. Instead of checking one image at a time, the app produces a page-level report showing which images fall below 300 DPI, which are fine and which cannot be determined from the file structure alone. This is the only method that scales to multi-page documents and produces evidence the print shop can review alongside the source file.
How to read a PDF DPI report
A useful report lists, per page and per image, the source pixel dimensions, the placed size in inches and the effective DPI on both the horizontal and vertical axes. Use the lower of the two axes as the conservative resolution. Mark images below the printer's threshold as evidence for discussion, not as automatic print failure: a 200 DPI photo may be acceptable in a newspaper insert but not in an art book. If the report cannot determine a value because the PDF stores no placement matrix, the report should say so instead of inventing a number.
Common PDF DPI problems and fixes
The most common problem is a high-resolution source image placed at a large size, dropping its effective DPI below the print requirement. Fix it by re-exporting the PDF with the image at its final placed size, or by replacing the image with a higher-resolution source. A second problem is an image with no placement matrix at all, which means the PDF viewer will assume 72 DPI; this usually indicates a broken export and should be re-exported from the source document. A third problem is a screen-grab or web image embedded at its native 96 DPI, which will look soft in print regardless of how the page scales it. Each problem has a different fix, so the DPI report must separate them instead of reporting one pass or fail.
Checklist before sending a PDF to print
Confirm the print shop's required DPI (commonly 300 for photos, 600 for line art), verify every page size matches the trim box, audit every image resource for effective DPI, export a page-level report with the source PDF's SHA-256, and share the report with the printer so any borderline image can be discussed before the press run. The report is evidence for a human decision, not a certificate that guarantees print quality.