How to quickly change and re-change a file type association in Windows XP?

Thank you for replying!
Unfortunately it does NOT work!...

What symptoms do you experience?  Perhaps you could you post the content of the three .reg files - there should be nothing confidential in them.

Strangely (at least for me), the *.reg are always similar:

Windows Registry Editor Version 5.00
 
[HKEY_CLASSES_ROOT\.pdf]
@="FoxitReader.Document"
"Content Type"="application/pdf"
 
[HKEY_CLASSES_ROOT\.pdf\PersistentHandler]
@="{DDA60C00-9937-453d-99E4-15967383ACE8}"
"Prev"="{3C3DCD30-E62D-49a4-A746-039334AFDE59}"
 
[HKEY_CLASSES_ROOT\.pdf\shell]
 
[HKEY_CLASSES_ROOT\.pdf\shell\open]
 
[HKEY_CLASSES_ROOT\.pdf\shell\open\command]
 
[HKEY_CLASSES_ROOT\.pdf\shellex]
 
[HKEY_CLASSES_ROOT\.pdf\shellex\PropertyHandler]
@="{67EB453C-1BE1-48EC-AAF3-23B10277FCC1}"
 
[HKEY_CLASSES_ROOT\.pdf\shellex\PropertySheetHandlers]
 
[HKEY_CLASSES_ROOT\.pdf\shellex\PropertySheetHandlers\InfoPage]
@="{B2F55D43-C7A4-4B7C-90D7-7A860DFA9F2A}"
 
[HKEY_CLASSES_ROOT\.pdf\shellex\{00021500-0000-0000-C000-000000000046}]
@="{B2F55D43-C7A4-4B7C-90D7-7A860DFA9F2A}"
                                              

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Doing what you said seems to be the same than doing nothing - registry is the same (at [HKEY_CLASSES_ROOT\.pdf]), and of course, nothing different happens... Maybe there are different registry values which really matters...

Hmmm... Do they all state FoxitReader.Document beneath that first entry?   If so, I suspect when you are switching PDF readers via the Open With dialogue, are you not enabling the "Always ..." option at the bottom of the dialogue before taking the next export (it is this which makes the change in the registry).

Yes, it is true - "FoxitReader.Document" always remains unchanged, despite I click on "Always ..." option.

asgarcymed--I do not know how much simpler this is, but once set up it should save a few clicks.
Right click any .pdf file|Open With|Choose Program.  If the 3 PDF viewer programs are already in the list, highlight one and click OK.  If they are not in the list already, click Browse and find the executable for one of the PDF viewers and add it to the list. Then highlight it and click OK.
Repeat the above for each of the PDF viewers.  
You should wind up with all the PDF viewers appearing in the initial Open With window--so you can directly click on one-- and you will not have to click Choose Program and go through the porcedure in that window.

After a lot of "blind trials", I found a way which is not technically perfect but it is close to the desired final feature:

1) First, I must manually "open with" a PDF file with each one of the 3 applications;

2) Next, I run "regedit" and jump to:

Windows Registry Editor Version 5.00
 
[HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\FileExts\.pdf]
"Application"="PDFXCview.exe"
 
; JUST NEED TO CHANGE THE NAME OF THE APPLICATION'S EXE FILE!...
; Names can be seen at:
; [HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\FileExts\.pdf\OpenWithList]
; "a"="PDFXCview.exe"
; "b"="FOXITR~1.EXE"
; "c"="Foxit Reader.exe"
; "d"="Foxit Phantom.exe"
; "e"="FOXITP~1.EXE"
; "f"="CABAReTStage.exe"
; "g"="AcroRd32.exe"
;: "h"="AltarsoftPdfReader.exe"
; "i"="PDFToolset.exe"

                                              

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Now what?... Is there any "registry expert" able to help me to improve this solution?...

To be a "perfect" solution, I would not need to manually "open with" a PDF file with each application; and I would have to indicate the full path of each PDF viewer (even portable versions, such as "Sumatra PDF"), like [pseudo-code]:

Ext = .PDF
Currently active = a
Open with option list:
a = "C:\Portable Apps\PDF\Sumatra.exe"
b= "C:\Program Files\Foxit\Foxit Reader.exe"
(and son on...)

On the other hand, I would like to set the PDF files' default icon to be the same of the application EXE file's icon (I would know what is the currently active viewer by looking at the icon of any PDF file, since I remember the icon of each application).

Do you have any ideas?

Thanks and regards