I am trying to find a way to attach some pdf files like brochures, manuals etc. but could not find how to do it.
I think that this feature is available but could not find it how to do. Is there any help?

I don't know the answer to your question but it makes me think that it would be a useful feature.
 
I have products that link to colour guides
- I guess I would put a link in the product description to an Acrobat file
 
Chris

or maybe add it as a free product thats downloadable after a sale?

adding as free product is not solves my problem. I am using viart as a product catalog not as a shopping cart. So, I must attach some downloadable files to the products. Many of the shopping cart systems have this feature. This feature also allows you to track the number of downloads of the files. I mean adding the links to the description is not also useful for me.

Ive looked at this and it does seem a shortcoming. Whilst I use links in the description it would be better to have an additional product "attachments" tab.
 
Perhaps this could be a "re-write" of the "images" one but allowing written media filetypes?
 
Im waiting for 3.7 to see what additions are included before suggesting this to support.

I believe you can use downloadable tab on edit product page. Along with each downloadable file you can set it preview with appropriate type 'Preview as download' and select where do you want to show this preview.
 
Hope it helps.