If the form is designed for on-line completion, then you should be able to use any browser that the web-form is designed to support.
If, however, your question is “how do I complete a downloadable form/document on my iPad”, this something entirely different. First, you will need to know the document type/format - and only will you be able to consider the App that you’ll require so as to be able to edit (i.e., fill-in) the document.
Remember also you’ll need to be able to either print or upload your completed document.
Unfortunately, you are unlikely to find a “universal” document editor for iOS. If you can provide some information about the document type/format that you need to edit, then contributors here may be able to offer some additional recommendations or advice.