PDF to PNG: resolution and file size

A PDF page is measured in points, and 72 points make one inch. Rendering at 72 DPI therefore gives one pixel per point; at 150 DPI it gives about two; at 300 DPI about four.

For an A4 page (595 × 842 points) the rendered sizes are roughly 595 × 842 px at 72 DPI, 1240 × 1754 px at 150 DPI and 2480 × 3508 px at 300 DPI. The converter renders at that scale rather than writing a bigger DPI number into a smaller image.

Every page is drawn into a canvas before it is encoded. At 300 DPI, one A4 page is about 8.7 megapixels, which is roughly 35 MB of raw pixel data before the PNG is written. Ten such pages held at once is more than many phones will allow.

This tool renders pages one at a time and releases each canvas afterwards, but the limits still exist. If a document fails, convert fewer pages at a time or choose 150 DPI.

PNG is lossless: the image is stored without the blocky artefacts JPEG adds around text and edges. The trade-off is size. A photo-heavy page will usually be much larger as PNG than as JPEG, while a page of text and line art often compresses well.

Higher resolution increases both dimensions and file size, and the increase grows faster than the setting suggests: doubling DPI roughly quadruples the pixel count.

A PNG of a page is a picture of that page. Selectable text, links, form fields and vector shapes become pixels, so they can no longer be selected, searched or clicked. If you need those, keep the PDF as well.

This is also not image extraction: the converter draws each page as it looks, rather than pulling out the original photographs embedded in the PDF file.

72 DPI is enough for a quick preview or a thumbnail. 150 DPI is a sensible default for screens, slides and documents. 300 DPI is for printing or for images that will be enlarged later, and it is worth converting only the pages you need at that setting.