The iOS email client sends single page PDF files as inline attachments, but some email clients do not support them.
Here is a potential workaround copied from a discussion online:
I narrowed this down to a problem in how the Mail app on iOS encodes the attachment when the user has an HTML signature set in their iOS Mail settings. If their signature includes an embedded image (the image data is included in the message) this causes the problem. When users delete their signature and replace it with a plain text or simple html signature it's all fine!