I can’t tell exactly what you’re doing, but here are two things to try: the first is when you send attachments, always tick the box that says “Send Windows-Friendly Attachments” that you see below the file listing when you navigate through the folders to attach the file. That will attach the file in a certain way in order for it to survive being emailed without damage. I know you’re not sending to someone with a WIndows PC, but still, try always ticking that box.
The other thing to try is zipping the file attachments. Zipping is a way of compressing the files to make them smaller, but it also ensures that they get transferred OK via email.
The way you zip a file is click once on the file in the folder or on the desktop (or in a folder) to highlight it. And then right click; if you don’t have a two button mouse or are on a laptop, hold down the Control key and click. Then select “Compress…” from that popup menu.
That will make a compressed zip archive of the file. The new file will have the same name, but have a .zip suffix, and the file icon will be a square with a tiny zipper down the front.
Then email that zip file as an attachment and tell your friend to double-click on it. The zip file will decompress into the original file.