Keepass 2
I recommend storing all of your passwords with Keepass 2, not just the passwords for these documents.
Here's a way to get Keepass to open the file and enter the password for you:
For each file, create a new entry in KeePass with the folder path as the UserName; e.g. C:\Inconspicuous\NefariousSchemes.docx
or C:\Users\SecretAgent\MissleLaunchCodes.xlsx
.
Put the password in the Password field.
For the URL, put this command for Excel files:
cmd://powershell.exe -command "(New-Object -comobject Excel.application).Workbooks.Open('{UserName}',0,0,5,'{Password}').parent.visible = $true"
...and this command for Word documents:
cmd://powershell.exe -command "(New-Object -comobject Word.application).Documents.open('{UserName}',$null,$false,$null,'{Password}').parent.visible = $true"

Once this is done, you can open any of these password-protected files by selecting the entry in Keepass and hitting Ctrl+U to run the command stored in the Url field.
