combine images into a PDF
A pixel has no physical size on its own: a 3000x4000 px photo can become a 31-inch page or an 8-inch page, depending entirely on the DPI chosen for the conversion. Combining several photos into a single PDF depends on that pixel-to-point math, on top of deciding whether each image becomes a page sized to itself or gets fit inside a standard A4 or Letter sheet.
At that DPI the original image is 3125 pt tall, bigger than the A4 usable area, so it gets shrunk (never enlarged).
At 300 DPI the same photo occupies a page almost 4x smaller than at 72 DPI, because the point-per-pixel value shrinks.
Page order is an index list kept separate from the encoding step, so reordering never goes through the canvas again.
Yes. Images become pages in the same order they were added. To reorder, remove an image and add it again in the desired position.
Because a pixel has no physical size on its own, the DPI is what converts pixels into inches. A higher DPI (like 300, used for print) makes each pixel worth a smaller fraction of an inch, so the final page is smaller; a low DPI (72, the screen value) turns the same photo into a much larger page.
No, fitting into A4 or Letter only shrinks the image when it is too large for the available margin; a small image is never enlarged past its original size, so it never comes out blurry on the page.
The limit is 500 pages, because each page keeps the full JPEG image in memory during file assembly, and a much larger batch risks freezing the browser tab before it finishes.
No, each image is JPEG-encoded exactly once and embedded in the PDF; changing the page order only moves that already-finished JPEG around, without decoding or re-compressing the image again.
The PDF is generated entirely in your browser. No image is sent to any server.