How do I attach file to text message

Assuming that the pdf is in the files app, locate the file, tap on it and then open the share sheet - the icon with the square and the up pointing arrow and tap on Messages. That will attach the file to a text message. Then type in the recipient name, etc, etc.

Sending through iMessage is no issue.

Probably you want to send a file to an Android user. This requires MMS services. Contact your Carrier to see if MMS services are enabled on your SIM.