Mail pdf attachment on iPhone

It’s possible the file is too big to be sent via Apple email. This link may be helpful, especially the section at the bottom, which I’ve copied below.

Add email attachments in Mail on iPhone - Apple Support

Note: If your file exceeds the maximum size allowed by your email account, follow the onscreen instructions to send it using Mail Drop. See the Apple Support article Mail Drop limits.

That is funny. But it changes the answer to your question. I thought you were using the iPhone Mail app to send the PDF. Since this is work related and you are using their specific software, you will have to ask them about your issue.

It is always up to the third party developer to make sure their software keeps up with Apple’s iOS updates. I frequently attach PDF files to email, using the Apple Mail app. I’ve had no problems with that.

Is the pdf a personal document or a work document?

Well … unfortunately, Apple cannot help with the software you and your partners wrote. Wish I could help but it is up to you. Maybe just use Gmail if that works?