Turning PNGs into a PDF wraps one or many images into a paged document at the page size and order you choose. It is what upload forms want when they refuse images, and what makes a set of screenshots into something that reads as a document rather than a folder. One point specific to PNG: transparency is flattened onto the page background, because PDF pages are opaque and there is nowhere for see-through to go. An image with a transparent background therefore arrives with a white one, which is almost always what you wanted but is worth knowing before you send a logo sheet to a printer. The images are embedded rather than re-encoded, so quality is preserved exactly and the PDF is roughly the sum of its inputs. It all runs in your browser, with nothing uploaded.
What happens to transparency
A PDF page is a physical sheet with a colour, and PNG's alpha channel has no equivalent in it. Transparent regions are composited onto the page background — white by default — at the moment the image is placed.
For screenshots and diagrams this is invisible and correct. It matters when the image was designed to sit on something else: a logo with a transparent background becomes a logo on a white rectangle, which will show against any coloured stock it is printed on. Where that matters, composite the image onto the intended background first.
Screenshots make good PDF pages
PNG is what every operating system produces when you capture the screen, which makes this the natural route for turning a sequence of captures into a walkthrough, a bug report or a record of something before it changed.
Screenshots are usually wider than they are tall, so a landscape page will use the space far better than portrait. Keeping them in capture order matters as much as with any document — drag the thumbnails rather than trusting the order the browser hands over.
Quality and size
PNGs are embedded as they are, so nothing is re-compressed and the pages look exactly like the source images. PNG is lossless, which is excellent for fidelity and unhelpful for size: a set of full-resolution screenshots produces a large PDF.
If the result is too big for an email or an upload limit, compress the finished PDF rather than accepting a lower-quality capture. Screenshot text tolerates moderate compression better than people expect, and it is easier to redo than the captures are.