72, 150 or 300 dpi: picking the right resolution
Resolution decides how many pixels come out of a PDF page and how large the PNG ends up. The three levels in the tool cover most situations.
What dpi means for an image file
A PNG has no physical size, only a number of pixels. When converting, dpi states how many of those pixels map to one inch of the PDF page. An A4 page becomes roughly 595 × 842 pixels at 72 dpi, 1240 × 1754 pixels at 150 dpi and 2480 × 3508 pixels at 300 dpi.
Rule of thumb: doubling the resolution quadruples the pixel count, and with it the file size.
72 dpi: screens and previews
72 dpi matches classic screen resolution. It is enough for quick previews, small graphics in a slide deck or an overview of many pages, and the files stay small.
Once someone zooms in or shows the image large on a high-density display, text turns soft.
150 dpi: the default for documents
150 dpi is a good balance between sharpness and file size. Text stays clearly readable, photos look clean, and the files can still be sent by email.
For screenshots of forms, invoices or contract pages, this level is usually the right choice.
300 dpi: print and crops
300 dpi matches typical print resolution. Choose it when the image will be printed, placed in a layout or enlarged substantially.
A single page can then reach several megabytes. With many pages it is worth selecting only the ones you need.
The limit is in the original
Higher resolution creates more pixels, not new information. If a scan was made at 200 dpi, a 300 dpi export is an enlarged version of the same image.
For scanned documents, check the scan quality first. If the source is soft, no output setting will fix it.