How do I remove the activation lock of my iPad without the purchase invoice?

Sorry, without a receipt there is no way for Apple to help you. You need to prove you actually own that iPad before Apple will help remove activation lock.

click here ➜ How to remove Activation Lock - Apple Support

If you don't know the Apple ID and password for it, you will need to try to recover them.

click here ➜ If you forgot your Apple Account primary email address or phone number - Apple Support

click here ➜ If you forgot your Apple Account password - Apple Support

Here are the Apple support pages that outline the Activation Lock - and the only mechanisms by which it can be satisfied:

Activation Lock for iPhone, iPad, and iPod touch - Apple Support

Turn off Activation Lock – Apple Support

As you will discover, unless you are the owner of the AppleID that was last used to activate the device, you’ll not be able to defeat or bypass the Activation Lock. With documentary evidence that you are the original owner (such as sales receipt showing the serial number), Apple may be prepared to assist - otherwise they and nobody here will be able to help you.

If you have the necessary documentation, Apple have introduced a portal though which you may be able to request release of an Activation Lock:

How to remove Activation Lock - Apple Support & start an Activation Lock support request.

AppleID

You may also need your AppleID and associated password. If these have also been forgotten, they can be recovered here:

https://iforgot.apple.com

To recover your credentials, you’ll need access to one of the following:

• Your primary email address mailbox that corresponds with your AppleID
• Any of the secondary/recovery email addresses that should be configured for your AppleID account
• Any of the trusted telephone numbers (fixed line or Cellular/Mobile) that are associated with your AppleID

More information about recovery of your AppleID and associated Password:

If you forgot your Apple ID - Apple Support

If you forgot your Apple ID password - Apple Support

The link below explains the steps you will have to follow to try to open this iPad. But I have to wonder if it's going to be worth going through all that for a 12-year-old iPad. You say you plan to use it for content, but please be aware that its OS, maybe too old for many of the apps you may want to use.

If you forgot your iPad passcode, use your Mac or PC to reset it - Apple Support