JPG to PDF
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A PDF is the right wrapper when several images have to travel as one document; a set of receipts, a passport photographed front and back, a signed page you photographed rather than scanned. Each image becomes a page, in the order you set.
How to turn JPGs into a PDF
JPEGs are embedded into the PDF byte for byte rather than being decoded and re-saved, which matters more than it sounds: a photo goes in at exactly the quality it arrived with, and the PDF ends up roughly the size of the photos added together. Anything that is not a JPEG: a PNG screenshot, a WebP; is drawn as lossless pixels so its transparency and sharp edges survive. If the result is too big for an email, the margin and page size do not help much; run it through the PDF compressor instead.
Questions
No. JPEGs are copied into the PDF untouched, so what you see is exactly the original file.
Because the photos are. A modern phone photo is three to six megabytes, and ten of them make a thirty-megabyte PDF. Compress it afterwards, or resize the photos first.
Yes. Each is embedded in whichever way preserves it best.
It makes every page exactly the proportions of its photo, so nothing is letterboxed. Useful for screenshots, less so for anything you will print.
Use the arrows next to each thumbnail. The top one becomes page one.
No. The PDF is written in this page by code that ships with it.