PDF reader and trusted identities Acrobat Reader has a nice feature "manage trusted identities" that allows to import CA certificates to validate the signature on a document.
Is there an option to define trusted identities in the Android version of the reader (or in any other free pdf reader for android)?
[Disclosure: I work for CoSign] 

I just viewed a digitally signed PDF in Adobe Reader for Android version 10.6.0.

Unfortunately, it does not appear that the app will verify the digital signature. It does display the graphical representation of the signature, but there is no Signature Panel and no message about the signature's verification status.

Hopefully they'll add that feature to the mobile version someday.

Please submit your own answer to your question if you find any Android software that will verify a digital signature. 

In the meanwhile, you could roll your own solution if you're willing to write an app. See the Signature API guide at www.arx.com/api for info.

Or you could use CoSign Web App, Box or Google Drive. All of those apps work in mobile and include integration with CoSign. By opening the doc in CoSign, you'd see the doc and its signature verification status. See screen shot below.