The accepted answer validates the position of 'also' in the text you quoted, but that text is wrong in various ways that are nothing to do with the placing, or use, of 'also'. You could have said 'the attached documents also include the certificates, as requested'.
'the documents are attached' is wrong in this context. You could say 'the documents that are attached' and that would not be wrong. The usual way of expressing that idea would be to say 'the attached documents'.
It has to be 'include' not 'includes' because 'documents' is a plural subject of the verb. If you had said 'the attachment ...' then you could use 'include' because 'attachment' is singular.