How multiple images become pages
Each image is one page, in list order. Drag to reorder; the first row is page 1. JPEG bytes are embedded as DCTDecode when the source is already JPEG — we do not decode-and-re-encode unless we have to, so a 400 KB photo stays about 400 KB on its page.
PNG is embedded as PNG (Flate). WebP is not a native PDF filter, so we convert WebP to JPEG inside the PDF. That is a re-encode: a 200 KB WebP might become a 280 KB JPEG page. EXIF orientation is applied so phone photos are not sideways.
Fit vs A4 vs Letter
Fit sizes the page to the pixel box of the photo. A 4032×3024 receipt becomes a wide page. Portals that expect “a letter-size PDF” can reject that. A4 is 210×297 mm (595.28×841.89 pt). US Letter is 8.5×11 in (612×792 pt). In those modes the image is fitted inside the sheet; you get letterboxing, not stretching.
Worked example: a 1080×1440 (3:4) photo on Letter with fit-inside sits in the portrait sheet with small side margins. The same photo on A4 is slightly taller relative to width, so the side gaps change. Pick the paper the printer will load.