Well in theory any change is allowed, but all changes invalidate the signature by definition. Some versions of Acrobat lock out more or less changes, it seems to change.
I don't know of any document that details what you are looking for, but here's a rundown.
First up, I assume you really mean is what changes do Acrobat/Reader allow, because there are many different PDF parsers and they may have different rules as to how to interpet a PDF signature. The PDF specification (ISO-32000) only details how to create the signature, it's then up to the viewing application to process what they find.
In the Acrobat world the only allowable changes are those that can be created outside of the signed data. That means data that is both external to the PDF structure and data that was created after the signature was created. If you try to modify the PDF structure (for example adding or editing a form field) that would not be allowed. Things that you can add without invalidating the signature are limited to filling out form fields and adding comments. Comments (e.g. sticky notes, drawing markups) and form data (e.g. text you type in, checkboxes you select, or addition signatures) you add post signing are not considered part of the PDF structure, but rather part of the annotation layer and get appended to the existing PDF file. Comments and form data do not invalidate a signature and are allow, and that's it, at least in Acrobat & Reader.