Why do I see margins?

Create Booklet does not modify any pages. If the app preview is showing margins around your document, this means that your document size in not exactly half* the size of the printing output paper size.

* Why “half”?

Let's say your printer maximum print output format is DIN-A4 and you want to print as booklet a 16 pages A4 document. Create Booklet will automatically layout for you 4 pages in one sheet of paper (two pages per side). To do so, the app will obviously scale down your original document so two pages can fit into 1 side of the sheet. As two A5 exactly fits into an A4, the booklet result will not have any margin issue. On the other hand, if your input format does not have the ratio of half the output paper, margins will appear.

Sadly, this is not applying on imperial system with 8.5" x 11" (US Letter) and so on, where margins will occur.

How to remove or minimize the margins?

If you can modify the source

In case you have designed your input document and doesn't mind to sightly modify it, you can make sure the document format is on the same dimension standard as the output format you'll use in Create Booklet to print.

In the metric system (DIN-A standard)

Two portrait A5 pages fit into one landscape A4 page. And of course the A5 page can be a shrunk down version of any DIN-A format, like A4. So it's quite simple, make sure to keep your design and printing paper format on the DIN-A format.

In the imperial system

Create input pages of 8.5" x 5.5" to print your booklet onto US Letter.
Create input pages of 8.5" x 7" to print your booklet onto US Legal.

If you cannot change your input format

3 tips that could help you dealing with margins:

Output paper example for US Letter/Legal sheets

US Legal is 8.5" x 14" and US Letter is 8.5" x 11". To make sure margins are reduced as much as possible: