Transparency and quality are kept
PNG is a lossless format, and so is the way it is stored in the PDF: the pixels are carried across unchanged, with no resampling and no JPEG compression. A PNG with a transparent background keeps it, so a logo or a cut-out still sits cleanly on the page. PDF readers normally show the page behind it as white.
How the pages are sized
Each page is created at the exact pixel dimensions of its image, one pixel per point, so nothing is cropped, stretched or padded. That suits screenshots and graphics. A high-resolution scan, though, becomes a very large page; it prints and zooms fine, but if you need A4 or Letter pages, a PDF reader’s “fit to page” print option does that without changing the file.
Size and other formats
Lossless means larger: photographs saved as PNG make big PDFs. If the file is too heavy to send, use Compress PDF, or convert photos to JPG and use JPG to PDF. To go the other way, PDF to PNG turns each page back into an image. The tool checks what is actually inside each file, so a JPG renamed to .png is caught and named rather than producing a surprise. WebP and iPhone HEIC are not supported yet.