The "Add Background" feature in Adobe Acrobat Pro can be used to place an existing letterhead file behind the content of a document or PDF. Depending on the layout of your letter vs the letterhead, this can be a one-and-done solution, or might require a bit of manual tweaking. This method is ideal for situations where you have completed letter that needs this formatting added after-the-fact.
- From Adobe Acrobat Pro, Open the document you'd like to apply the letterhead to.
- Choose the Edit PDF tool, click on More at the far-right of the top toolbar, and choose Background > Add.
- In the Add Background panel, select File for the Source, and pick your letterhead file via the Browse button.
- Click OK to confirm and apply the selection.
Note: Letterheads for this method can be any of several common image formats, or a PDF. If the letterhead is something like a Microsoft Word file, you can open it in Word, choose Print, and select "Adobe PDF" as your printer. That resulting file will then work for the above process in Step 3.