What no recompression means for a JPG
A JPG file is a small header with tables, followed by the compressed picture. A PDF can hold exactly that, because it has a built-in filter called DCTDecode, which is the JPEG decoder. So a converter does not have to touch the picture at all. ClipLime writes the JPG file into the PDF as the page's picture, without its hidden camera details, and draws it on the page. A PDF viewer then decodes it the same way your photo app would.
We checked this by taking the picture back out of the PDFs made on this page and comparing it with the file that was added. The compressed picture data was identical, byte for byte, for baseline, progressive, grey, colour-profile and CMYK files. Only the hidden EXIF block with the camera details and the location is gone.
This matters because JPG loses a little detail every time it is saved. A converter that opens a JPG and saves it again, even at a high setting, adds a second round of that loss on top of your camera's, with faint blocky marks around edges and text. Copying the file avoids it, and is fast, because nothing is decoded or encoded.
Phone photos, rotation and colour
Cameras and phones usually do not turn the pixels of a photo held upright. They save the picture as the sensor read it and add an orientation tag, from 1 to 8, to say how to show it. Photo apps obey the tag. PDF viewers do not apply it to a picture inside a PDF, so a converter that ignores the tag gives you portrait photos lying on their side.
This tool reads the tag and applies it through the page: the photo is drawn turned (or turned and mirrored, for the mirrored values) when the page is made, and the JPG data stays as it is. The rotate buttons add your own quarter turns on top.
A grey JPG stays grey, which keeps it small. A JPG with a colour profile carries the profile into the PDF as the picture's colour space, so viewers that use profiles show the colours the photo was meant to have. A CMYK JPG, the kind that comes out of print workflows, is stored as CMYK. PDF viewers turn CMYK into screen colours in their own way, so the colours can look a little different from what your photo app shows.
Location data, size, and when to choose Smaller file
A JPG usually carries more than the picture: the camera model, the date and, if your phone recorded it, the GPS position. Before a photo goes into the PDF, this tool cuts that data out of the file itself, byte for byte, so the picture is exactly as it was and the PDF does not tell anyone where it was taken. In our check, the picture inside a PDF made from a photo with a GPS tag carried no tag at all, and its picture data was identical to the original's. The result says how many photos had hidden data removed. If a file is damaged in a way that makes its hidden data unsafe to read, it is stored as it is and the result says that it still holds a location. A photo that Smaller file reduces is saved again and carries no metadata either. To clean photos that are not going into a PDF, use Remove EXIF data.
The PDF is as big as the photos together. Photos from a modern phone are commonly several megabytes each, so ten can make a PDF too large for email. A 12 megapixel photo of 4000 by 3000 pixels on a landscape A4 page is drawn at about 363 DPI, while 200 DPI is plenty for screens and office printers. With Smaller file the same photo becomes 2205 by 1654 pixels, about 30 percent of the pixels, saved again as JPG at quality 84. With Fill and crop and Original quality, the whole photo stays inside the file and the page edge only hides the cut-off part. Use Original quality for a print shop or an archive, and Smaller file for sending.
JPG files that need a different treatment
A JPG that looks cut short, because a download did not finish, is refused with a message, and the other photos still go in. A JPG of more than 100 megapixels is refused too, because a browser tab cannot hold it safely.
A few rare JPG kinds cannot be stored as they are, because PDF viewers do not agree on them: 12 bit files, and files that use arithmetic or lossless coding. If your browser can open such a file, it is saved again as JPG at high quality and the details say so. If not, the file is left out and its row says why. A file with a .jpg name that is really a PNG is recognised by its content and handled as a PNG.