How the pages are sized
Each page is created at the exact pixel dimensions of its image, so nothing is cropped, stretched or letterboxed. A mix of portrait and landscape photos produces a PDF with a mix of page sizes — which is usually what you want when the source is a camera roll rather than a document scan.
Does it re-compress my images?
No. JPEGs are embedded exactly as they are, with no quality loss and no re-encoding. PNGs are embedded losslessly too, which means a PDF of PNG screenshots can be large — convert them to JPG first if size matters more than perfect fidelity.
Which formats work?
JPG and PNG. The tool checks the actual file contents rather than trusting the extension, so an image saved with the wrong suffix is caught and named rather than producing a corrupt PDF. HEIC from iPhones is not supported yet — most photo apps can export a copy as JPG.