When an image of the page is what you actually need
Going from PDF to image is the right move more often than people expect. Slide decks and social posts want a picture, not an attachment. A web page that needs to show a certificate or a menu needs an image. Systems that accept photographs but not documents — many older HR and government portals among them — need each page as a file they recognise.
It is also how you make a page visible where PDFs are awkward. A PDF pasted into a chat becomes an attachment somebody has to download; an image appears inline and gets read. If the goal is for a person to actually look at the thing, an image usually wins.
Each page becomes its own image file, so a ten-page document produces ten images. If you only want page four, it is faster to extract it first and convert the single page than to convert everything and hunt through the output.
Resolution, and why 150 DPI is usually right
A PDF page has no inherent pixel dimensions — it is described in points, and it is rendered into pixels at whatever density you ask for. This is why the resolution choice matters so much and why the same page can come out as either a crisp document or a blurry one.
At 72 DPI an A4 page is roughly 595 by 842 pixels, which is fine for a thumbnail and too soft for reading. At 150 DPI it is about 1240 by 1754, which is comfortably readable on any screen and the right default for almost every use. At 300 DPI it is around 2480 by 3508 — genuine print quality, noticeably larger files, and worth it only if the image will actually be printed or if someone needs to zoom into fine detail.
Higher is not automatically better. A 600 DPI render of a text page produces an enormous file that looks identical to the 300 DPI version on any screen, and image-hosting services will often recompress it back down anyway.
The one-way door, and choosing a format
Converting a PDF to images is lossy in a way that is easy to underestimate. Text becomes pixels. The result cannot be searched, cannot be copied from, cannot be read aloud by a screen reader, and cannot be edited. Converting back to PDF gives you a PDF containing pictures of text, not the document you started with.
So keep the original. Send the image where an image is wanted, and keep the PDF for everything else. If you need both a viewable image and a searchable document, that is two files, not one conversion.
On format: JPEG suits pages that are mostly photographic and produces small files, at the cost of soft edges around text at lower quality settings. PNG is lossless and keeps text edges sharp, which makes it the better choice for pages of text, diagrams or anything with hard lines — at the cost of a considerably larger file.
Converting a long document without drowning in files
A hundred-page PDF becomes a hundred image files, which is rarely what anyone wants. Before converting, decide honestly whether you need every page as an image or whether you need three specific pages. Extracting those pages first and converting only them is faster, produces a manageable set of files, and saves you from renaming ninety-seven images you will never open.
When you genuinely do need all of them, the files are numbered in page order, so they sort correctly in any file browser as long as the numbering is zero-padded. If you are feeding them into another system, check how it sorts: some tools order files as text rather than as numbers, which puts page 10 immediately after page 1 and quietly scrambles your document.