Practical

The file name does not decide the order

Phones name photographs by time, and time is not always the order the document should read in.

Arrange the images before you build. A receipt set usually wants date order; a contract photographed page by page wants page order, which is the same thing until one page was reshot because the first attempt was blurry — and that reshot page now sorts to the end.

Size

A modern phone photograph is several megabytes. Twelve of them make a document too large for some mail systems. The ceiling here is 100 MB per file and 50 files per task, which is rarely the constraint; the recipient's inbox usually is.

If the document is being sent rather than kept, reducing the photographs on the phone before you build is the simplest fix and the only one that happens before the pages exist.