The Browser shows the sample_pdf file instead of using browser plugin.
Generally, a web application provide the pdf directly to download over browser force to download pdf and open office file.
Suppose user does not have any plug in browser to show pdf file in browser and does not have acrobat reader on machine read file. Then user require to install acrobat reader in machine.
Design you website in such a way so that your website will able to show pdf / open office (.odt) files directly on web browser instead of forcing user to download file. You can use viewerJS for this task.
Structure of downloadable file:
you can also follow viewerjs.