PDF and image files can represent the same visual page, but they do not have the same structure. Converting between them is therefore more than changing a filename extension.
PDF to image means rasterizing a page
When you convert PDF pages to images, the page is rendered onto a pixel grid. Text, vector graphics and other PDF elements become part of that raster image.
The result is convenient for previews, thumbnails, sharing a page as a picture or using it in software that does not accept PDF. But selectable text and vector scalability may no longer be available in the image.
Resolution controls the amount of detail
Rasterization needs a target resolution. Too few pixels can make small text blurry; an unnecessarily high resolution creates larger files without visible benefit for the intended display size.
The guide Image resolution: pixels, dimensions and quality explains the relationship between pixel dimensions and display quality.
JPEG, PNG and other image formats
The best output depends on the page. JPEG can be efficient for photographic content but uses lossy compression. PNG is often better for sharp interface elements, diagrams or text-heavy graphics where preserving edges matters.
See PNG, JPEG, WebP and AVIF compared for a broader comparison of image formats.
Images to PDF: assemble pages without pretending they become editable text
Convert images to PDF is useful for scans, photographs of documents or a set of image pages that should be distributed as one file. The resulting PDF provides page structure and convenient sharing, but placing an image inside a PDF does not automatically turn the photographed words into selectable text.