I never knew these commands even existed, I always just want to the registry. I wonder which versions of Windows they shipped with, I only have 10 available to test here.

assoc

Displays or modifies file extension associations

ASSOC [.ext[=[fileType]]]

  .ext      Specifies the file extension to associate the file type with
  fileType  Specifies the file type to associate with the file extension

Type ASSOC without parameters to display the current file associations.
If ASSOC is invoked with just a file extension, it displays the current
file association for that file extension.  Specify nothing for the file
type and the command will delete the association for the file extension.

ftype

Displays or modifies file types used in file extension associations

FTYPE [fileType[=[openCommandString]]]

  fileType  Specifies the file type to examine or change
  openCommandString Specifies the open command to use when launching files
                    of this type.

Type FTYPE without parameters to display the current file types that
have open command strings defined.  FTYPE is invoked with just a file
type, it displays the current open command string for that file type.
Specify nothing for the open command string and the FTYPE command will
delete the open command string for the file type.  Within an open
command string %0 or %1 are substituted with the file name being
launched through the assocation.  %* gets all the parameters and %2
gets the 1st parameter, %3 the second, etc.  %~n gets all the remaining
parameters starting with the nth parameter, where n may be between 2 and 9,
inclusive.  For example:

    ASSOC .pl=PerlScript
    FTYPE PerlScript=perl.exe %1 %*

would allow you to invoke a Perl script as follows:

    script.pl 1 2 3

If you want to eliminate the need to type the extensions, then do the
following:

    set PATHEXT=.pl;%PATHEXT%

and the script could be invoked as follows:

    script 1 2 3

Usage

So for the PDF extension you can query using assoc .pdf which returns .pdf=AcroExch.Document. Then you can take that file type and run ftype AcroExch.Document which gives you AcroExch.Document="C:\Program Files (x86)\Adobe\Acrobat 10.0\Acrobat\Acrobat.exe" "%1".

>assoc .pdf
.pdf=AcroExch.Document

>ftype AcroExch.Document
AcroExch.Document="C:\Program Files (x86)\Adobe\Acrobat 10.0\Acrobat\Acrobat.exe" "%1"

(I still remember parts of my autoexec.bat and config.sys days so I changed my prompt using prompt $g.)