MLA in-text citation of documents written by an organization, like UNESCO?
The document you refer to is written by an organization or a corporate author (UNESCO).  The general rule in such cases is, you treat this as an ordinary source written by an author named UNESCO.
From the Purdue Online Writing Lab, on listing work written by an organization in your works cited list:

A corporate author may include a commission, a committee, or a group that does not identify individual members on the title page. List the names of corporate authors in the place where an author’s name typically appears at the beginning of the entry.

The same general idea applies to in-text citations of sources written by an organization or corporate author:

When a source has a corporate author, it is acceptable to use the name of the corporation followed by the page number for the in-text citation. You should also use abbreviations (e.g., nat'l for national) where appropriate, so as to avoid interrupting the flow of reading with overly long parenthetical citations.

Here's an example, courtesy of Red Deer College Library:

Parenthetical Citation
(Natl. Research Council 55)
Works Cited
National Research Council. Beyond Six Billion: Forecasting the World's             Population. Washington: Natl. Acad., 2000. Print.