How to link an image with a PDF? Actually, I have a content type with 2 fields, image field (multiple) which will contain some pictures and an attachment field (accept only one file) which is file of type PDF.

I have a view where I put my gallery and each picture opens in a colorbox. The PDF file (actually global pdf for all images) is only a button.

There was a change and the new requierement is: "For every image that goes up, must have its own PDF. Every image should have its own associated file."

Another requirement is, if the image is opened on a colorbox also appears a button (in the colorbox) which displays the PDF associated with the corresponding image.

Any idea???
Sorry for my bad english.
You can create and use https://www.drupal.org/project/field_collection to have a new field containing an image field and a file field for your pdf.
You set the field in unlimited. 
Problem is that you might have to reupload every picture manually or write an update script that does the conversion for you.

Then for the colorbox popup with your button, have a look at https://www.drupal.org/project/entity_popup