How to avoid adding renditions for pdf uploading into DAM?

When I upload pdf file to cq5 I see following structure in my crx:

enter image description here

This structure looks strange for me. I don't understand why do I see png renditions here.
How can I use these renditions?

My current vision that these renditions couldn't be helpful. Thus I want to achieve that after uploading PDF files into DAM these renditions doesn't generate.

Can you provide steps for this?

enter image description here

after my changes in workflow: enter image description here

before my changes:

enter image description here

update 2

I set dimensions follows: enter image description here

but I see old result after pdf and jpg uploading

Solution

The purpose of these renditions is, as per their name, that they can be used to display thumbnails of the documents — e.g. you may want to display the front cover of each PDF if in a gallery section.

The thumbnail is created by the "DAM Update Asset" workflow (http://www.example.com/etc/workflow/models/dam/update_asset.html; until v5.6) in its Thumbnail creation step — I'm not sure that you can configure this to skip certain file types.

To get around this, you could remove this step and replace it with an Create Web Enabled Image step — this will allow you to specify MIME-types to skip over in its arguments panel:

enter image description here

Alternatively, you could write a bespoke Process class which would use ImageMagick to generate thumbnails but ignoring PDFs.