If you're emailing, see here:
https://www.theanswerbank.co.uk/Technology/Question1832187.html
However your secondary post above suggests that you're using Safari (or possibly an alternative web browser) to send the attachment via a website. In which case, there should be a button somewhere on that web page marked 'Attach' (or something similar). Tapping that button should open a dialogue box, where you can look for the attachment on your iPad and then tap on it to choose and send it.