You need to make your pages 10mm larger in each direction (so that you have 5mm on top, botton, left and right edge). You can do this in Adobe Acrobat by using the Crop tool:
Open up your document, then right-click on a page in the Page Thumbnail pane, right-click and select "Crop Pages...". This will bring up the crop dialog. An A5 page is 148x210mm, so we need to get to 158x220mm.
In the bottom left corner of this new dialog you should see "Change Page Size". Select custom and then enter 158 and 220. Make sure that the "Center" checkbox is checked.
Then select to apply to all pages in the Page Range box on the right side.
Now go back into the Crop dialog and set your bleed box to 5mm from the four edges.
Karl Heinz Kremer
PDF Acrobatics Without a Net
PDF Software Development, Training and More...
http://www.khkonsulting.com