How to convert a PDF to JPG

PDF measures pages in points, where 72 points is one inch. An A4 page is 595 × 842 points, which is 8.27 × 11.69 inches; Letter is 612 × 792 points, or 8.5 × 11 inches. Multiply the inch dimensions by your chosen DPI and you have the output pixel dimensions: A4 at 150 DPI is about 1240 × 1754 pixels, and at 300 DPI about 2480 × 3508.

Choose by how the image will be used, not by what sounds high. Something displayed in a web page at 800 pixels wide gains nothing from a 2480-pixel render except download weight. Something that will be printed at full size needs 220 DPI or more, because printing a 150 DPI raster at page size is visibly soft.

A useful check: if you will be zooming in to read something, you need more DPI than if you will be glancing at the whole page. The cost is quadratic — doubling DPI quadruples the pixel count and roughly the file size — so the step from 150 to 300 is a bigger commitment than it looks.

JPEG was designed for photographs. It works by approximating blocks of the image in a way that exploits how human vision handles smooth tonal variation, which is why it is excellent for a photo and poor at a hard black-on-white edge. The letter shapes in a document page are nothing but hard edges, so JPEG puts faint grey halos around them — most visible at small type sizes and low quality settings.

PNG is lossless. Every pixel comes out exactly as rendered, which makes it the correct choice for text pages, tables, screenshots, charts and anything with flat colour areas. Flat colour also compresses well in PNG, so a text page as PNG is often not much larger than the JPEG, and sometimes smaller.

The rule that holds up: photographs and scans of photographs to JPG, everything that was born digital to PNG. If a page mixes both — a report with a photo — decide by which part matters more, or render at a higher JPEG quality to protect the text.

Text stops being text. The words become pixel patterns that happen to look like letters, so the output cannot be searched, selected, copied or read by a screen reader. If someone needs to find a phrase in it later, they cannot, and getting it back requires running OCR on the image — recognition of your own document, with the error rate that implies.

Everything structural also goes: links become flat pixels, form fields become pictures of boxes, bookmarks vanish, and vector artwork that was resolution-independent becomes fixed at whatever DPI you chose. Zooming into a rasterised diagram shows pixels where the original would have stayed crisp at any magnification.

This is exactly why conversion to images is sometimes the goal — a page as an image cannot have its text edited or extracted casually, which suits a proof or a sample. Just be clear about which of those two situations you are in, because the conversion is one-way.