magicfilter is a common tool to manipulate files for printer. It analyzes the type of the file to be printed and invokes a filter script to make appropriate pretty-printing. These scripts are invoked from lpd that is started from /etc/init.d/lpd that is started from init. Thus, the path is that of init. That does not contain /usr/bin/X11!
You might want to insert printing of PDF files to magicfilter. It is possible to do this by using /usr/bin/X11/xpdf. Now you must remember to insert full directory path to the file name because magicfilter would not find it otherwise. Most programs used in magicfilter do not need full path, because they are on /bin or /usr/bin.
You may use PRINTER environment variable to show what is the printer that you are using. However, you may notice that in some cases in X applications it is sometimes lost.
You must remember that if the X session is started from XDM, the window manager has never evaluated your shell login scripts. All the X applications that you have started from xterm have your PRINTER variable. However, if the same application is started from menu or window manager button, it does not contain your PRINTER variable.
In some cases this can be inherited to an even lower layer: for example a Netscape helper application can have or have not your PRINTER definition.