ACZoom

This seems like a user (browser) configurable item - certainly the best way to do this would be using the soon-to-arrive CSS3 target-new features and just styles to do this. It does require a minor enhancement to linkfilter to create a specific style name for pdf files - but that fits in the current functionality, so is not much of a conflict.

Other options - like accepting a target="new" text as is, or with a code, within the l: tag is somewhat more obtrusive. It would imply that writers would need to permanently embed that text in the node text, hard to edit or remove etc. Or linkfilter could store user options in a database to turn things like this on/off - that is a major change to linkfilter, which has tried hard not to require any database (causes problems on Drupal upgrade, etc). So, a lot to consider to make this work well, not sure yet of the best way to implement this, within linkfilter without the new CSS3 stuff.