Unable to send a 150MB PDF file via my email account on my iPhone

There is no email provider in the world that will send or receive a 150 MB attachment. Most limit attachments to 10 MB, a few (Apple included), to 20 MB.

You can send larger attachments using Apple’s Mail Drop feature. Once enabled, when you send a large attachment your phone uploads it to iCloud, then puts a link in the email to download the attachment. Here is more information on Mail Drop→Add an email attachment in Mail on iCloud.com - Apple Support

This is a user-to user forum. If you want to contact Apple use the Get Support link at the top of this page. Things that can prevent an app from recognizing a Wi-Fi connection include the possibility that the Wi-Fi router does not have a connection to the Internet even though there is a Wi-Fi “fan” in the status bard, or you are using a VPN connection that does not terminate in a location where the email server “exists”. You should also be able to send email using cellular data. There is no limit to cellular data for email, but, as I said, there is a definite size limit from all email service providers.

If Mail Drop is not working for you, or not very suitable for you, or if you don’t like how the large file stays embedded in Mail.app, then you may consider third party alternatives, such as WeTransfer (sign-up optional). They’ll host large files for a week (longer for a fee) and send email to recipient and sender with a download link. I like it for temporary stuff.