Hi fox256,
With regard to messages using your iPhone, the possible options are:
1) Text, video or images sent to other Apple devices. These messages deliver via your data connection through the Apple iMessage service when enabled and configured. Should there be a failure to deliver over the iMessage service, these will default to SMS or MMS. These message bubbles appear in blue on your device.
2) A Text-only message to a single non-Apple device under a character limit threshold (usually 160 characters) deliver via your cellular connection through your cellular carrier network as SMS to the cellular carrier network of the recipient for delivery. These message bubbles appear in green on your device.
3) Messages containing video, images, multiple recipients and some text only messages which exceed the SMS threshold to non-Apple devices (or group messages containing at least 1 non-Apple device) deliver via your cellular connection through your cellular carrier network as MMS to the cellular carrier network(s) of the recipient(s) for delivery. These message bubbles appear in green on your device.
Based on the information you provided, these messages would be transmitted as MMS and rely upon the cellular carriers for both you and the recipient for transmission. Any blocks, restrictions or feature mis-matches in those connections may cause a failure for the delivery or receipt of said messages. Your carrier and/or the carrier of the other individual will need to investigate the failure of these messages to transmit through their services to find and resolve the matter.
This information is covered in What is the difference between iMessage and SMS/MMS? You can also use the details from If you can't send or receive messages on your iPhone or iPad to verify your device settings are correctly configured before contacting your carrier to continue. You may need to verify with them that they support the transmission of pdf files over MMS as you indicate that is the only concern you've encountered.
Regards.