A silent install of doPDF is possible. I have just finished building a silent installer for automated deployment and can confirm that it works very, very well.
doPDF utilizes Inno setup for the installer. Inno is a free installation utility which can be downloaded here: jrsoftware.org/isinfo.php An Inno unpacker utility can be downloaded here: sourceforge.net/projects/innounp/files
That will unpack the files and a portion of the original install script. From there you will need to determine the command line of the program run within the installer called dopdfin6.exe. The following page will show how to capture that command line from a running full doPDF installer.
With that information and the Inno setup compiler, you can then simply build a new installer with no dialogs shown and call it with the Inno standard /VERYSILENT switch and you're all set.
Please do not use this to violate the doPDF EULA or to otherwise injure Softland.