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From Oxygen XML Editor/Author

A publishing template can be used for PDF output from the DITA Map PDF - based on HTML5 & CSS transformation scenario (or from the DITA PDF - based on HTML5 & CSS transformation scenario).

The Templates tab in the transformation scenario dialog box displays all the templates that are available in your template gallery. To use a particular template in the transformation scenario, simply select it from this tab and then continue configuring the transformation using the other tabs to suit your needs.

Result: All the templates contained in your template directory will be displayed in the preview pane along with all the built-in templates.

From Command Line

You can use the simpler form, meaning you only use the pdf.publishing.template parameter to point to the *.opt (publishing template) file:
dita.bat 
	--input=map\test.ditamap" 
	"-Dpdf.publishing.template=full_path_to_template_dir/my_template.opt" 
	--format=pdf-css-html5 
	...
Or use the two parameters to indicate the folder containing the publishing templates and the name of the publishing template file relative to that folder:
dita.bat 
	--input=map\test.ditamap" 
	"-Dpdf.publishing.template=full_path_to_template_dir" 
	"-Dpdf.publishing.template.descriptor=my_template.opt"
	--format=pdf-css-html5 
	...
Tip: You can also start the dita process by passing it a DITA OT Project File. Inside the project file you can specify as parameters for the webhelp-responsive transformation type the WebHelp-related parameters.