Why transparent preview? I press Space at any document. 
I get transparent preview which makes difficult to read anything about the document.
I think this is not a feature of the latest OSX 10.10.3. 
I have reduced transparency maximally in the settings. 

Example



How can you prevent the transparency of previewing files?
You are right that this isn't a feature of OSX. It looks like it is showing you the code of PDF files instead of rendering them as PDF files. My guess is you've either installed a QuickLook plugin that is doing this, or you've installed an application with a QuickLook plugin that is handling files instead of the default plugins.

There are three folders for QuickLook plugins. Please check these folders for any plugins that you've installed yourself that could be doing this. "/System/Library/QuickLook", "/Library/QuickLook" and "/Users/YOURUSERNAME/Library/QuickLook".

Alternatively you can run the command qlmanage -m plugins to see a list of plugins on your system. This will tell you the plugins from these folders, but also includes the plugins that come bundled with applications. You could even filter it to just show those that affect PDF files with qlmanage -m plugins | grep pdf.

Let us know how you get on.