Please, it's not possible. TMemo requires only text, nothing more.
Converting a PDF is not what you want. Even pictures are not allowed to load into TMemo the right way.
If you look at your mail messenger, you see a listview with only the filename. When this listbox is filled, messenger know there are attached files to sent. It takes the file into a stream, convert it into text and send the attachment to the receiver.
I created a simple mail messenger in our application with indy to do it for the user without knowing how it works.