Apple already compressed once. Do not punish the file again.
iPhone stills use a modern codec to keep texture in a small camera file. The failure mode online is a second lossy hop: downscale for “speed,” JPEG at a low slider, then PDF as a dumb wrapper. You did not preserve anything. You made a social thumbnail with extra steps. This spoke is the photographer-minded default: start from the Camera Roll master, not a WhatsApp forward; embed at native dimensions; skip gratuitous recompression. The PDF will be heavier than a crushed email photo. That is the trade. Proofs, fabric, product texture, and archival packets need those bytes. Local encode means no unknown server pipeline rewriting pixels. We still cannot invent detail that a screenshot never had. If you need a true lossless raster master, export PNG or TIFF, then place that in a layout app. PDF here is a document that carries the still faithfully enough for review and print prep — not a RAW file.