Quit Acrobat if its already running.
With the June 2021 release of Acrobat version 21.005.20048, the PDF document security password must be at least six characters long for both the ways you can set a document password:
This enhancement is not applicable to Acrobat 2020 and Acrobat 2017.
It's recommended to set at least six characters long passwords to protect your PDF documents for security reasons. If you still want to use a password that's fewer than six characters long, follow any of the workarounds below.
In Acrobat, go to Help > About Adobe Acrobat and check if your application is updated to the latest version 21.005.20054. If not, download and install the update manually from here.
After updating your application, do the following:
Open the registry editor - go to Run (Windows menu + R), type regedit.exe in the Open field and click OK.
In the registry editor, based on your installed version of the product, go to the version-specific location listed below, and create the key FeatureState at the location; the key may not be present by default.
Under FeatureState, create a key of type DWORD and name it bDisableMinPasswordLen. Change its value to 1 (one).
Close the registry editor.
Navigate to the folder Library/Application Support/Adobe/Acrobat/DC/Preferences/.
Open the com.adobe.acrobat.FeatureState.plist file in any plist editor. If you don't find the plist file, create a file of type Property List with the name 'com.adobe.acrobat.FeatureState' using any plist editor.
Create the following entry in the FeatureLockDown key:
Lock Path: FeatureLockdown
Type: 0 (bool)
Default Value: 1
Save the com.adobe.acrobat.FeatureState.plist file.
You can set a document password that's fewer than six characters long using the Action Wizard tool. After creating the action, you can reuse it to set a short password on any PDF document.
In the Create New Action dialog box, from the Choose tools to add column, choose Protection > Encrypt. The Encrypt tool is added to the Action steps to show column on the right. Click Encrypt > Specify Settings.
When you run the action on a PDF document, Acrobat will allow you to use fewer than six characters long passwords.