You get a palette with an option called 'Fullscreen' to maximize the editor (as shown below).
This maximized editor provides with a number of option to edit and format the text. Select the 'Source edit' option from the ribbon as shown below:
Replace the code with an iframe container to include a file, where the source can be a pdf in DAM Assets or even a link. Once done click the same Source Edit option to preview the file. You can do similarly for gif files.
Once done, close the editor using the 'Minimize 'option available on the top right side of the screen.
Now, Preview the form.
The container will require a plugin to render a pdf, like in my case it's Adobe Reader. You may also get the option to download, print and zoom the pdf based on the plugin.
Sample Form preview
Embedding a Video
All the steps would remain the same except you need to use the below code in the source edit of the text Editor
Is this functionality no longer supported after AEM 6.5? The iframe works when I select "minimize" and even appears as expected on the adaptive form, but when I click the checkmark to save changes on the static text component, the iframe is there but empty.