The "problem" seems to be on the acroread.postinst file, lines 145-171. This is what Canonical's package contains:

case "$1" in
    configure)
        ReadInstallDir="/opt"
        MakeLibGnomeSpeechLinks "$ReadInstallDir" intellinux 2>/dev/null
        MakeLibLDAPLinks "$ReadInstallDir" 2>/dev/null

        #rm -f /usr/bin/acroread 2>/dev/null
        #ln -s "$ReadInstallDir/Adobe/Reader9/bin/acroread" /usr/bin/acroread 2>/dev/null

        #rm -f /etc/bash_completion.d/acroread.sh 2>/dev/null
        #ln -s "$ReadInstallDir/Adobe/Reader9/Resource/Shell/acroread_tab" /etc/bash_completion.d/acroread.sh 2>/dev/null

        #rm -f /usr/share/man/man1/acroread.1.gz 2>/dev/null
        #ln -s "$ReadInstallDir/Adobe/Reader9/Resource/Shell/acroread.1.gz" /usr/share/man/man1/acroread.1.gz 2>/dev/null

        #InstallIcons "$ReadInstallDir"
        #Canonical does not want us to associate MIME types because they say they do it themselves.
        #install_mime --novendor "$ReadInstallDir/Adobe/Reader9/Resource/Support/AdobeReader.xml"

        #for i in application/pdf application/vnd.adobe.xfdf application/vnd.fdf application/vnd.adobe.xdp+xml application/vnd.adobe.pdx application/fdf application/xdp application/xfdf application/pdx; do
        #    PATH="$ReadInstallDir/Adobe/Reader9/Reader/intellinux/bin:$PATH" xdg-mime default "AdobeReader.desktop" "$i" >/dev/null 2>&1
        #done

        #install_desktop_menu --novendor "$ReadInstallDir/Adobe/Reader9/Resource/Support/AdobeReader.desktop"
        #Canonical does not want a Desktop icon.
        #install_desktop_icon --novendor "$ReadInstallDir/Adobe/Reader9/Resource/Support/AdobeReader.desktop"
        #PATH="$ReadInstallDir/Adobe/Reader9/Reader/intellinux/bin:$PATH" xdg-icon-resource forceupdate >/dev/null 2>&1
        "$ReadInstallDir/Adobe/Reader9/Browser/install_browser_plugin" -silent -global "$ReadInstallDir" >/dev/null 2>&1

And this is what Adobe's package contains:

    configure)
        ReadInstallDir="/opt"
        #MakeLibGnomeSpeechLinks "$ReadInstallDir" intellinux 2>/dev/null
        MakeLibLDAPLinks "$ReadInstallDir" 2>/dev/null
        
        rm -f /usr/bin/acroread 2>/dev/null
        ln -s "$ReadInstallDir/Adobe/Reader9/bin/acroread" /usr/bin/acroread 2>/dev/null
        
        rm -f /etc/bash_completion.d/acroread.sh 2>/dev/null
        ln -s "$ReadInstallDir/Adobe/Reader9/Resource/Shell/acroread_tab" /etc/bash_completion.d/acroread.sh 2>/dev/null
        
        rm -f /usr/share/man/man1/acroread.1.gz 2>/dev/null
        ln -s "$ReadInstallDir/Adobe/Reader9/Resource/Shell/acroread.1.gz" /usr/share/man/man1/acroread.1.gz 2>/dev/null
                
        InstallIcons "$ReadInstallDir"
                # Cannonical does not want us to associate MIME types because they say they do it themselves. Then so be it, atleast for 1
build.
        #install_mime --novendor "$ReadInstallDir/Adobe/Reader9/Resource/Support/AdobeReader.xml"

        for i in application/pdf application/vnd.adobe.xfdf application/vnd.fdf application/vnd.adobe.xdp+xml application/vnd.adobe.pdx app
lication/fdf application/xdp application/xfdf application/pdx; do
            PATH="$ReadInstallDir/Adobe/Reader9/Reader/intellinux/bin:$PATH" xdg-mime default "AdobeReader.desktop" "$i" >/dev/null 2>&1
        done
                
        install_desktop_menu --novendor "$ReadInstallDir/Adobe/Reader9/Resource/Support/AdobeReader.desktop"
                # Cannonical does not want a Desktop icon. Then so be it, atleast for 1 build.
        #install_desktop_icon --novendor "$ReadInstallDir/Adobe/Reader9/Resource/Support/AdobeReader.desktop"
        PATH="$ReadInstallDir/Adobe/Reader9/Reader/intellinux/bin:$PATH" xdg-icon-resource forceupdate >/dev/null 2>&1
        "$ReadInstallDir/Adobe/Reader9/Browser/install_browser_plugin" -silent -global -user "$ReadInstallDir" >/dev/null 2>&1
    ;;

IMHO, the for loop at lines 164-166 should be uncommented in order to associate Adobe Reader 9 with .pdf files properly.