If you want to embed attachments you have already uploaded do the following, it's a lot easier that it seems and takes no time at all to do this.
1. Add all the attachments and text and click post.
2. Open the post up like you were reading it normally.
3. Click the attachment so it appears full size.
3. Right click on the image and save the image URL.
4. Edit the your post.
5. Paste the URLs where you want it to appear in text (on it's own line).
6. Highlight the text and click the image icon. Or manually type out the img bbcode tags.
7. Save the edited post.
Example of img bbcode tags around a URL.
[img]http://oi68.tinypic.com/cnhg3.jpg[/img]
Posting documents and PDFs of normal conversation into forum threads is punishable by death, don't ever do it. It is considered very poor internet etiquette, especially as most forums allow you to embed attachments after posting. Same goes for posting dozens of pointless images and videos and doing it in consecutive posts for no reason, that is called Spam. Attaching super high resolution images to the forum when it's not needed is also poor form, it wastes the servers bandwidth and people will notice if you make a habit and start to hate you.