I typed "this is a big old office". I understand the Arabic words technically stated big and old, however it's more common in English to say "big old office " rather than "big and old" which is usually reserved for specifically stressing two adjectives that don't commonly go together or with three or more adjectives....
If you look carefully at the Arabic and the English phrases, the word (office: مكتب) is indefinite. In Arabic it does not have (AL), and in English it is preceded by (a), and attributive adjectives. Anyway, since it is about typing in English, checking that the Arabic sentence does not define (office) then we must have (a). Your sentence defines "office" by virtue of using "this" which would be translated as: هذا المكتب instead.