In iOS 13 the Files app has a useful utility for scanning files accessed via the ellipsis menu in the top-right of the screen.
The scanned document will be saved in your iCloud Drive folder. It will normally
be quite large, possibly too large for Gmail’s 25Mb attachment limit. In that case, use
ps2pdf to reduce its file size:
$ ps2pdf scanned-file.pdf smaller-file.pdf
ps2pdf is provided by the
package installed with:
$ brew install ghostscript
This has worked well in my experience but there are alternatives on this Stack Overflow page.