In order to digitally sign documents, Adobe Reader has to be configured first.
This happens in a central Adobe configuration file and covers the DFN PKI certificate chain, the DFN-PKI time stamp service (for a confidential time stamp)
as well as the integration of the DFN PKI LDAP directory service in Windows.
All further instructions require the installation of  Adobe Reader X (or Adobe Acrobat X).

1. Start Adobe Reader and go to Edit-->Preferences.

2. First, make sure that Adobe is familiar with the T-TeleSec root certificate required by TU Dresden.
3. Click the Signatures option under Categories on the left. In the Identities and Trusted Certificates section, press the More... button. The Settings for digital IDs and trusted certificates window opens.

4. In the navigation menu on the left, click Trusted Certificates. Make sure that a certificate with name and issuer T-TeleSec_GlobalRoot_Class is missing in the displayed list. You can download the certificate T-TeleSec_GlobalRoot_Class_2.crt (if this won't work, please right-click on this link and open this link in a new tab) via this link or directly from the DFN PKI web pages.
5. Then select the Import button above.

6. Another window will open, go to Browse... and open the downloaded certificate. An entry "T-TeleSec GlobalRoot Class 2" is now listed in the Contacts section.

7. Continue by clicking Import.

8. A dialog box "1 issuer certificate(s) imported" appears. Confirm this with OK. The list of trusted certificates now includes an entry "T-TeleSec_GlobalRoot_Class" as the name and issuer of the certificate.

9. Select the certificate and press the Edit button.

10. Now check the Use this certificate as trusted root and Certified documents options. Close the window by clicking OK. 

11. Click the Security option under Categories on the left. On the right side of the window, check Load security settings from a server. Enter in the URL field:

https://tu-dresden.de/zih/ressourcen/dateien/dienste/arbeitsumgebung/e_mail/dateien/tu_dresden_adobe_security

12. In the option box Settings must be signed by: select Allow Any Certificate and click on Update Now.

13. Confirm all following windows.

14. Now the configuration of your certificate is still missing. Please navigate to Signatures and click on More...

15. There, select Digital IDs from Windows and then your certificate on the right side of the window, if your certificate does not yet exist, you can import it via Add ID left from Usage Options.

16. In the menu, please activate Use for signing. You can now sign documents with your certificate.