Each month I send an invoice to my clients. Included in that invoice are photos of any receipts for expenses incurred (taxis,etc.). I typically just take a photo of these recipes flat on my desk and then save them as PDFs and drag those PDFs into the main invoice PDF in Preview to create a single file with both the invoice and all the receipts.
This month, I went to send the invoice to the client and Gmail balked at me as the file size was too large. The PDF was 22MB, but I guess once Base64 encoded and S/Mime encrypted it was over the 40MB-ish limit it appears we have on Gmail.
I remember seeing a filter when saving a PDF from preview called 'Reduce File Size'. Sure enough, that reduced the file size down from 22MB to 250KB. But the images of receipts were so heavily compressed that you could not read some of the numbers on them.