Why put images in a PDF at all
A PDF is a container that keeps order and travels as one file. That is the whole benefit: a set of photographed receipts, a scanned form, or pages of a document captured on a phone are all far easier to send, print and archive as one paginated document than as a folder of loose images an email client will reorder.
The images are embedded as-is rather than re-encoded, so the JPEG data in the document is byte-for-byte what you supplied. No quality is lost in the wrapping, and the PDF is roughly the sum of the images it contains.
Order, orientation and size
Pages appear in the order the files are listed. Each page is A4, turned to whichever orientation matches its image, with the image scaled to fit and centred — so a mix of portrait and landscape photographs produces a document that turns with them, and nothing is ever cropped to fit a page.
If the result is larger than you want to email, the images were probably large to begin with — compress them first, which works much better than compressing the finished PDF. For PNG sources use PNG to PDF, and to pull pages back out as images, PDF to JPG.