PNG vs JPG vs WEBP for PDF Pages
The output format is the first choice the converter asks you to make, and it has more effect on the result than anything else except resolution. Here is what actually differs between the three, framed around the kind of content a PDF page usually holds.
Lossless and lossy
PNG is lossless. The file it produces can be decoded back into exactly the pixels that were rendered, with nothing discarded. JPG and WEBP are lossy: they analyse the image, throw away detail the eye is least likely to miss, and store an approximation. That is how they achieve their much smaller sizes.
This distinction matters more for documents than for photographs. Lossy compression works by smoothing out high-frequency detail, and a sharp black letter on a white background is about as high-frequency as an image gets. That is why heavily compressed JPGs of text pages develop faint grey halos around the characters, an artefact usually called ringing.
PNG
Choose PNG when the page is mostly text, tables, diagrams or line art, and when you care about edges staying clean. It is also the safe choice if the image is an intermediate step rather than a final product, because repeatedly re-encoding a lossy format degrades it a little each time, while PNG can be opened and re-saved indefinitely without loss.
The cost is size. PNG compresses by finding repetition rather than by discarding detail, so a page with a lot of varied content compresses poorly. At high scales a single page can easily exceed several megabytes.
JPG
JPG is the most universally supported image format in existence. If your images are going somewhere old, unusual, or out of your control, JPG will work. It also handles photographic content extremely well, so scanned pages and image-heavy brochures are a natural fit.
For text-heavy pages, keep the quality setting high. At 0.9 or above the ringing artefacts are usually invisible at normal viewing sizes. Below about 0.7 they become obvious on screen, particularly around small type. Note also that JPG has no transparency, so any transparent region of a PDF page is composited onto a solid background.
WEBP
WEBP is the newest of the three and generally the most efficient. At an equivalent perceived quality it commonly produces files noticeably smaller than JPG, and it handles both sharp edges and photographic gradients competently, which makes it a good default for mixed pages.
Its weakness is compatibility outside the browser. Every current web browser decodes WEBP, but plenty of older desktop applications, print workflows and corporate document systems still do not. If you know where the file is going and it is a modern environment, WEBP is usually the best answer. If you do not know, it is a risk.
A rule of thumb
For text documents you want to keep or edit, use PNG. For anything going onto the web where you control the pipeline, use WEBP. For scans, photos and situations where compatibility is unknown, use JPG at a quality of 0.85 or higher. When in doubt, convert one page in two formats and compare the results before committing to a hundred.
Whichever you pick, remember that the format choice interacts with resolution. A high-quality JPG at a generous scale will almost always look better than a PNG at a scale that was too low to begin with.