One document instead of twelve attachments
The reason to convert JPG to PDF is almost never the format itself. It is that you have several images and need to send them as one thing: a set of receipts, a signed form photographed page by page, a portfolio, a set of screenshots for a support ticket. A PDF holds them in order, opens the same way on every device, and arrives as one attachment instead of twelve.
That is why the ordering controls sit next to each row rather than being buried in a settings panel. The order is the document. Photograph a four-page contract out of sequence and fixing it here takes two clicks, where re-doing it in a photo viewer means starting again.
Page size, and when it matters
Fit-to-image gives every page the shape of the photo on it. Nothing is cropped, no white bands appear at the edges, and the file is as small as it can be. This is the right choice for anything that will be read on a screen.
A4 or US Letter gives every page a standard printable size and centres the image inside it, rotating to landscape when the photo is wider than tall. Use one of these when the document is going to a printer, a fax number, or an office system that expects a paper size. The margin control matters here too: a printer that cannot reach the edge of the sheet will otherwise clip whatever sits there.
What this does not do
It does not run text recognition, so the words in a photographed page stay part of the picture and cannot be searched or selected. It does not deskew, sharpen or brighten a photograph of a document. And it does not compress an already small JPEG any further, because re-encoding one that has already been through a lossy pass costs quality for very little size.
Everything runs in your browser. Files are never sent to a server, nothing is stored, and there is no queue behind other people's documents.