Sorry, but you can't keep people from downloading copies of documents  and
still give them access to the metadata  of the library  unless you install
rights management server. The document  is stored as an attachment and is
therefore metadata. So if you have enough security to see the metadata then
you can read  the document.

You can use another list for metadata  , e.g. you have a document  lib doclib
A with 3 metadata columns (name,author,date). Then u can accomplish readonly
list in this way “

1. Create a list say LIST A with same columns (name,author,date , or any
other optional columns). as in doc lib doclib A

2. Attach a Item added/updated/deleted event handler on your doc lib
doclib A . and insert/update/delete item in LIST A (only metadata fields)

3. Give readonly access on LIST A and remove permission  from doclib
A