When you browse to a document of a content-type that Mozilla cannot view internally, Conkeror normally prompts you for an action to take on the content. You can customize this on a per-mime-type basis to automatically do what you like by registering a content handler:

content_handlers.set("application/pdf", content_handler_open);

content_handlers is a mime-type table associating mime-types and mime-type patterns with content handler functions. The following handlers are predefined for your convenience:

Save the document, prompting for path.
Open the document, promting for a command.

Open the document in the viewer registered in external_content_handlers for this mime-type, or prompt if none.

Prompt for a command, and run it with the URL of the document as its argument.
Copy the URL of the document.
The default prompt---the same as if no handler was defined.

If you set a give null instead of a content handler to content_handlers.set, it will unset the handler for the given mime type.

You can also use the mime-type wildcard "*". A mime-type of "*" will match any mime-type. A mime-type of, for example, "application/*" will match any mime type whose major type is "application".