This is how I would phrase the body of each email [with alternatives in brackets]:
1. It was not very easy [was quite difficult] to find the documents you asked me about, but I managed to do it. I have attached the file to this email [The file is attached to this email].
2. I have read the article. [If you really want to use your phraseology (which sounds impersonal to a native English speaker), then write it as "the article has been read']
3. Exactly the same as for 2. above. The timescale is irrelevant.
4. This is a slightly more difficult one, raising the question of who closed the windows, and when. You could write either "Yesterday, when I got home, I saw that the windows in my room were [not "was", because windows are plural] closed", or "...had been closed".
Using "were" is quite neutral, and leaves the matter of who closed the windows open: either yourself, before you left; or someone else, after you had left your home.
Using "had been closed" gives more of an impression that someone other than you had closed the windows after you had left home.
PS. It is fine to use "by someone", as in "It was done [or "It is being done"] by someone".
Student or Learner