I ended up doing a B/W scan at a lower resolution (my lawyer said it would be fine) and the document shipped okay - and before your post. However, I did run a test using your solution and, yes, that colour JPG would have shipped just fine. Thanks for the tip.
Also if you use Photos to "send to email" a picture, it will always give you several options on what size of image to send, can scale it down to a reasonable size for you automatically on-the-fly.
Under at least OS X 10.10 (and beyond, and maybe even before) an e-mail composed in Apple Mail has the option via a pop-up menu in the e-mail itself to choose the size (small, medium, etc.) of an attachment. The corresponding size in KB or MB is also shown. This may not be the case with PDF's (I don't recall).
That will work for JPEGs and other "lossy" graphic formats, but it not available for PDFs.
PDF is a Page Description Language that uses defined tags to tell the PDF interpreter in the computer or printer how to format the document. This can even include a description of the typeface (font) in case the document is printed on a machine that does not have the font installed. It is all that descriptive material that increases the size of the PDF file. If you want to see what the PDF code looks like open a PDF file with a pure text editor (one that does not try to interpret the PDF) such as TextEdit, TextWrangler or BBEdit. PDFs can be compressed using Zip, 7Zip, RAR, TAR, and other text compression utilities, but that has to be done outside of Mail but you may find the compressed file is larger than the uncompressed version because of compression overhead.
