The thing most converters get wrong
A PDF can hold a JPEG natively. The format has a filter called DCTDecode
that means, literally, "the bytes that follow are a JPEG, hand them to the JPEG decoder".
So converting a JPG to a PDF should cost you nothing at all.
A lot of tools do it the lazy way instead: decode the JPEG to pixels, draw it onto a canvas, encode a new JPEG at whatever quality they picked, and put that in. It works, and it also throws away detail you will never get back. Run the same photo through five such tools and you can see the damage.
With Quality on Original, this tool skips all of that. It reads the dimensions out of the JPEG header, writes the header into the PDF, and copies the file body across. The only case where it cannot is a CMYK JPEG, which is rare outside print workflows and gets converted properly instead of coming out with inverted colours.
What each quality setting does
| Setting | What happens | Use it when |
|---|---|---|
| Original | JPEGs copied in unchanged. Nothing else is touched more than once. | Almost always. |
| High | Re-encoded at quality 92. Visually identical on photographs. | You need every page normalised to the same encoder. |
| Smaller file | Quality 80. Roughly half the size on a typical photo. | Emailing a batch, or an upload form with a size cap. |
| Smallest file | Quality 65. Visible on text and hard edges. | The form says 2 MB and nothing else worked. |
Getting the page order right
Browsers do not sort dropped files. What you get is whatever order the operating system handed over, which for a folder selection is often creation order and occasionally nothing recognisable at all. If the pages matter, check the numbered list before you export and move things with the arrows.
A trick that saves the fiddling: rename the files 01.jpg,
02.jpg and so on before you select them, and add them one at a time in that
order. Tedious once, reliable forever.
JPG, JPEG, and the other spellings
They are the same format. .jpg exists because MS-DOS only allowed three
letter extensions and .jpeg did not fit. .jpe and
.jfif turn up occasionally and are also the same thing. All of them are
handled identically here.
If your files came off an iPhone they are probably .heic rather than JPG,
which is a different problem with a different answer: see
HEIC to PDF.