Netscape needs to run Acrobat Reader as an application.  To do this,
edit your Preferences in Netscape:

In Netscape, go to the "Edit" menu and select "Preferences."  A window
should pop up.  

Click on the "Navigator" category and go to "Applications."  Scroll
down the list of Applications until you find the line "Portable
Document Format."  Click on that line to highlight it.  

Select the "Edit" button on the bottom and another window will pop up.
This is what the window should say:

   Description: Portable Document Format
   MIMEType: application/pdf
   Suffixes: pdf
   
   Application: /mit/acro/arch/@sys/bin/acroread %s