What you are proposing is somewhat unusual as an iPad (and its operating system) is not well designed to act as a server. Nevertheless it should be possible though with some restrictions (sandboxing).
You need an app which will act as a file server on the iPad. A file server might use one of a number of protocols for sharing. Lets list a few and any apps I know of:
SMB (Windows file sharing): I am not aware of any iOS app which acts as an SMB server.
WebDAV which uses http: Apps like GoodReader or Documents by Readdle include a webDAV server, there may well be others.
FTP: At one time there was an iOS app called 'FTP Server', but it is no longer in the App Store. I don't know of any other.
Remember that even if you use Readdle's Documents or some other server app, Apple's sandboxing requirements mean that any file transfers can only be to and from the application's Documents folder on the iPad.
A further restriction, confirmed by @HorstWalter's testing, is that the webDAV servers above only remain active whilst the app is in the foreground.