Drop a few photos into a Word doc, save as PDF, and watch the file size jump from 200 KB to 80 MB. PDF images are simple in concept but full of quiet decisions about resolution and compression. Here's the gist.
Resolution: pixels per page
Images in PDFs are usually stored at their original resolution. A 12-megapixel phone photo placed on an A4 page contains far more detail than the page can ever show. The file carries every pixel; the viewer just downsamples to fit. If you don't plan to print at poster size, that data is wasted weight. Compress PDF downsamples images sensibly — print quality (300 DPI), screen quality (150 DPI), or web quality (72 DPI).
Compression: JPEG vs lossless
PDFs use different compression for different image types. Photos get JPEG compression (small but lossy). Diagrams and screenshots often use PNG-style lossless compression (bigger but crisp). Sometimes the export tool guesses wrong — JPEG-compressing a screenshot makes the text look fuzzy. Better tools let you choose, but the simple fix is to insert the image as the right type from the start (export screenshots as PNG).
Why scans bloat so much
Scanned documents are pure images, often at 300–600 DPI in colour. Even a black-and-white page becomes a 24-bit colour photo. Switching scans to black-and-white mode or 300 DPI at scan time saves 80% of the file size. After the fact, compress PDF does similar work, and Flint can OCR the result so the text becomes searchable.
FAQ
Will compressing make my logos look bad?
Default presets preserve text crispness and lightly compress photos. Logos sometimes show slight softening — for branding-critical PDFs, use the lighter compression option, or replace logo images with vector versions in the source.
Can I extract an image from a PDF?
Yes — most editors let you select and save individual images. Convert PDF to JPG extracts page-by-page; for specific images, use Flint's editor to select and export.
Are PDF images full quality?
It depends entirely on what was put in. PDFs preserve what they're given. A blurry phone photo stays blurry. A high-res scan stays high-res unless compressed.
Images are usually 90% of a heavy PDF. Run yours through compress and watch it lighten with no visible difference.