Some pdfs are protected.. copy&paste of content doesn’t work on them.
An option then is to do a screenshot of the interesting part/page/etc. and post that instead. That’s also the most sensible if you want to show drawings.
See here and in the following posts:
PS: to add images to a post there is a upload button.. 3 to the right of the link button or you can just drag&drop the image/content onto the post and it will upload automatically (if the content is accepted by the filter).