Image to PDF, on this site, means JPG and PNG files written into one PDF. Each file is one page. The order of the list is the order of the pages. That is the whole feature. It is the same converter as JPG to PDF and PNG to PDF, so a search for any of those three phrases lands on a form that does the same thing. Formats outside those two are not guessed. A live photo, a HEIC, or a WebP should be exported as JPEG or PNG before you add it.
The pictures are read in the browser and embedded with pdf-lib. They are not uploaded. A scan of a document, a screenshot of a receipt, or a set of homework photos stays in the tab until you download the PDF. There is no account. The page size is the image at 96 pixels per inch, reduced only when it would exceed A4, and never stretched. Words inside a photograph stay pixels. This is not OCR, and it is not a photo editor.
If the result has to sit beside a PDF you already have, download it and open merge PDF. If the result is too heavy to email, open compress PDF afterwards and expect the pages to become JPEG pictures. Keep the original images until you have checked the PDF, because this page does not keep a copy for you and cannot split the PDF back into separate image files.
Add files in reading order. A multi-select may sort by file name, which is right for scan-001, scan-002 and wrong for a camera roll named by the clock. When the order matters, add one file, then the next. The page count after success is the check that every image was included.