The Adobe Approved Trust List (AATL) is used to distribute and
maintain a list of trustworthy digital certificate issuers for Adobe
Acrobat and Adobe Reader.
Acrobat 9 or Reader 9 and later versions automatically download
the AATL from Adobe every 90 days for updates. The feature is enabled by
default and no user invention is required.
The 90-day timer is active if you open a digitally signed
document, sign a document yourself, or access the signature
functionality of the product.
If you never perform these actions, the
AATL will not be downloaded to your computer until you do so.
Afterwards, when the user receives a digitally signed document
from a signer whose digital certificate can trace its chain back to a
root on the AATL, that signature will automatically be trusted.
If you want to verify that the AATL is enabled, choose Edit
("Acrobat" on Mac) > Preferences > Trust Manager and be sure that
the “Load trusted root certificates from an Adobe server” checkbox is
selected. You can click the “Update Now” button in that same dialog box
to download the latest version of the Trust List from Adobe.
For more information visit http://www.adobe.com/security/approved-trust-list.html