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If you want to add any custom css code to the extension, then add them to the file components/com_edocman/assets/css/custom.css through Edocman Configuration page -> Tab: Custom CSS so that they will not be lost when you update Edocman in the future.
Edocman is a standard Joomla MVC extension so it works based on standard Joomla rules. Joomla allows you to overwrite output of any components and modules from the template (Checkout more about this here: http://docs.joomla.org/How_to_override_the_output_from_the_Joomla!_core) so Edocman also allows you to do that easily.
To overwrite output of product details page, you need to copy file: components/com_edocman/view to folder templates/your_template/com_edocman/view and modify this file from the template.
For example:
root -> components -> com_edocman -> view -> document -> tmpl -> default.php
and the overriden will be in
root -> templates -> pronostar -> html -> com_edocman -> document -> default.php
Once the file is putted in the template folder, it will be used instead of the file in the Edocman component. So in the future, if you update Edocman, the your modifications will be not lost.
You should use overwrite output from the template in the case you just have some minor changes on a few of pages.