Hi yoyo921,
We appreciate you reaching out to us about sending a file via email. Rest assured that we’ll do our best to assist you or get you pointed in the right direction.
Are you and the receiver using an iCloud email? Are you and the receiver using the Mail app? We ask because the the Mail app and iCloud emails will not change the format of the file.
Also, how is your client opening the file? We ask because it is also possible that the app or program that your client is using could be converting the file format.
Below are some articles that we'd like to share to help ensure that the attachments on your email are added correctly.
Work with email attachments on iPhone
Mail Drop limits
Add attachments to an email on iPhone
Are you experiencing a similar issue If you attempt to download the file from your sent folder?
These details can help us in isolating the issue.
Cheers!