PDF Extension

If you feel safe in using Regedit, then have a look at this registry keys:

KEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\PropertySystem\PropertyHandlers                    (x64)
HKEY_LOCAL_MACHINE\SOFTWARE\WOW6432Node\Microsoft\Windows\CurrentVersion\PropertySystem\PropertyHandlers       (x86 on x64 system)

here you'll find a subkey for each file extension manager registered for Windows Explorer.

If you open the .pdf subkey you should find this content:

(default)   REG_SZ    {AEE2E439-D294-41D8-BE62-34D154070307}

where {AEE2E439-D294-41D8-BE62-34D154070307} is the GUID of PDFPropertyExtension property handler.

If you want to register it for .pdf1 file extension then create a new .pdf1 subkey and fill  it with the same content as the existing .pdf.

Or you can export .pdf key content (right click on .pdf -→ Export), save to .reg file, edit file content to replace .pdf -→ .pdf1, save .reg file and finally double-click the file to import it.

You're right, there's also this key that defines the properties available for each extension:

HKEY_LOCAL_MACHINE\SOFTWARE\Classes\SystemFileAssociations\.pdf

You need to duplicate it to a .pdf1 key

I've found what's wrong: the extension applies two "validations" on each file:

• filename must end with “.pdf”
• file content must begin with PDF fingerprint “%PDF-”

The first one was added in the early ages to avoid opening unparseable files but… it's redundant.
Windows Explorer already calls the extension only for configured file extensions.

Attached here you'll find a version without that extra check; please test it and report any issue 😉