Some versions of Adobe Reader may display an error message stating that the certificate is invalid, even though it is not.
It then looks like this at the top when you open the file in Adobe:
And under the paper clip it looks like this:
This is a known bug that you can read more about in the Adobe community. This means that Adobe only validates against it´s own trust list AATL, while our certificate is EU issued and must be validated against EUTL.
There is a support thread where you can check for updates. You may also check this thread or a workaround in this thread.
This does not mean that there is anything wrong with your signed file, but you may want to double check by also validating it on this validation page.
Please make sure that your settings are correct and that you have the latest version of Adobe.