Avoid the user is asked to download the files I am very new to Drupal sorry if my question is very basic or common.
I am creating a web site using Drupal 7 and I am using Tack da files module to tack files which are available for download.

My problem is that I have many PDF files in my site available for download, and the sites shows a popup window to download some of those PDF files. I do not want that popup window, but the site showing the PDF.

Why does it offers the user to download the files, instead of showing them?
By listening to it , it sounds that your browser does not have PDF plugin installed , that is why it downloads file instead of opening.

The same thing can happen to lot of your user, solution to that is Mozilla Project pdf.js

Drupal also have a module for this PDF Module, check that out and see if that helps.