Thanks for all the replies. The case of a composer who is still alive is quite clear. He/she decides what to do with his/her work and I need his/her permission, right?
In the other case, my question was more in general, about some future (or past) work I'll do (or did), rather than with the specific Brahms score. Thus, I put the Brahms example because I'm working on the 4. Intermezzo, Op. 76 (http://imslp.org/wiki/8_Klavierst%C3%BCcke,_Op.76_%28Brahms,_Johannes%29
), but I have only a single photocopy of the score, so I'm not really sure about the copyright. Anyway, it looks the same as the uploaded one except from the layout (Op. 76, No. 4 as subtitle missing, bar numbers added,...). So, I guess I can upload my typesetted score as a .sib file. Anyway, it was only an example, and probably a bad one. I'm sorry.
But I can give a better example: I have a version of the Introduction and Variations on a theme by Mozart, Op.9 (Sor, Fernando) (http://imslp.org/wiki/Introduction_and_Variations_on_a_theme_by_Mozart,_Op.9_%28Sor,_Fernando%29
) and my version is edited by Brian Jeffery, copyright Tecla Editions 1996. I checked my version against the original one and there are differences like "Andante largo" instead of "Andante" or some notes added to embellish the melody. So, if I understood your replies correctly, I can typeset the score in Sibelius/Finale, removing the additions by Brian Jeffery so it will have only the same information as the original, and upload the .sib/.mus file to IMSLP. Also, if the .pdf file wasn't already uploaded, I could upload that too. Is that right?
Oh, and one more question. What about arrangements or transcriptions? Everybody can upload his own transcription of a piece? For example, I see a transcription of the Fantasía No. 10 (Mudarra, Alonso) (http://imslp.org/wiki/Fantasia_No.10_%28Mudarra,_Alonso%29
). If I made a different transcription from the original tab, can I upload it? Is there any kind of requirement to upload a new transcription?