As I gather you've gathered, images typically take up the most amount of space in a PDF and there's only so much compression one can do with them before they have so much degredation from the compression as to make them, well, ugly.
If the issue is how to email a large file, than there is an alternate solution to splitting the file into sizeable bits: use a file-transfer website. What this entails is that you go to a file transfer website and after signing in, creating passwords and maybe a few other handshakes, you upload a file to their servers, then once uploaded, they send a message to the recipent that there is a file waiting for them. Once the recipient receives the message, they can then download the file from the file transfer website.
For files of your size (the whole file, not the pieces), it's free. For very large files (over gigabytes), you need a professional membership. Another thing that may require professional membership is if you want the link to last more than a week. Again, what you have can easily be done for free.
One service I've used recently is " https://wetransfer.com ." Please note I am not officially recommending them, it's just that I've used them recently and they did a fine job. What I've observed over the years is that these companies tend to come and go and policies occasionally change so what's good new may not be in a week or month or year. YMMV.
One thing I do recommend is that you send an email to the recipient that you will be sending the file via this method so that they know that they are not receiving spam and that it's safe to click on the link. These concerns do come up when one doesn't know who or what "wetransfer" is and why they are sending the person links to click on.