Pdf91 — Working with PDF files outside paid software | pdf91.com
Points, millimetres and pixels describe the same page three ways; the DPI setting is the exchange rate between them.
PDF measures pages in points, 72 to the inch. A4 is 595 × 842 points — 210 × 297 mm, or 8.27 × 11.69 inches. US Letter is 612 × 792 points, so the two formats differ by about 0.33 inches in width; a document printed on the wrong one shows it at the margins.
Convert to pixels and the file-size mystery explains itself. A4 at 300 dpi is 2480 × 3508 pixels, about 8.7 megapixels per page. That is the raw material a scanner produces, and the reason a handful of scanned pages becomes a multi-megabyte PDF.
On narrow screens, swipe or scroll the plate sideways.
Print shops think in three resolution tiers. 150 dpi is fine for a proof on screen; 300 dpi is the standard for photos in print; 600 dpi is needed for small line art or halftone text. Sending 150 dpi artwork to a photo print job is the classic cause of a soft, disappointing result.
Resolution also decides whether machines can read the page. OCR needs roughly 300 dpi on the source scan; below about 200 dpi, character recognition accuracy drops sharply and the text layer comes out as confusable fragments.
For full-bleed work, add bleed to the page geometry: printers normally ask for 3 mm beyond the trim line on every side, so a 210 mm wide A4 page becomes 216 mm of artwork before trimming. The trim size stays A4; the file you send is larger.
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