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If there isn't any placeholder already set, you could drag and drop the ADV directly on a page. For this purpose, you must set the option “Allow adv allocation directly in page” from the Administration Panel.
If you already have the Advertising PDF into the System, you can drag and drop it into the Plan. This way, you will be able to see the preview in the flat Plan.
NB: Make sure that advertising previews are enabled under the menu item “Views”.
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