Converting images to PDF wraps them into a single paged document, in the order you arrange them, at a page size you pick. It is the standard answer to an upload form that will not accept photographs, a receipts claim that wants one file rather than fourteen, or a set of phone photos of paperwork that should arrive as a document rather than a gallery. Each image becomes one page, scaled to fit the page size while keeping its proportions, so nothing is stretched. Photographs are placed as-is rather than re-encoded, which means the quality that comes out is the quality that went in and the resulting PDF is roughly the sum of the images. Everything happens in your browser: photographs of your passport, bank statements or medical documents are never uploaded to anyone's server, which for this particular conversion is usually the point.
Page size, orientation and fitting
A4 and Letter are the sensible defaults because they are what printers and document workflows expect. Each image is scaled to fit inside the page while keeping its aspect ratio, so a landscape photo on a portrait page sits centred with space above and below rather than being cropped or distorted.
Where the images are consistently landscape — slides, wide scans, spreadsheets photographed sideways — choosing a landscape page removes that empty space and makes the document much better to read. Mixed orientations are the awkward case, and are usually better solved by rotating the images first so the document is at least consistent.
Order, and why it is rarely what you expect
Browsers hand over multi-file selections in an order that is neither the order you clicked nor reliably alphabetical, and phone photo names sort in ways that surprise people once the counter rolls over. Drag the thumbnails into the order you want before converting.
For a document that was photographed page by page, this is worth two minutes of care: a bank statement or a signed agreement with pages out of sequence reads as carelessness to whoever receives it, and reassembling it afterwards means starting over.
File size and quality
The images are embedded rather than re-encoded, so the PDF is approximately the sum of the input files plus a small overhead. Modern phone photos are several megabytes each, which means a dozen pages of paperwork can produce a document too large for the form you are filling in.
The fix is to resize or compress the photographs before converting, or to compress the PDF afterwards. Paperwork does not need 12 megapixels — a page of text is perfectly legible at a fraction of that, and reducing the images first gives a much better result than heavy compression later.