My advice is to replace the select elements with RadDropdownList which has better integration with RadClientExportManager and not style the regular select elements with RadFormDecorator.
As to the odd rendering: can you please provide a video and reproduction steps?
As to the full screen when exporting the content to PDF:
My advice is to increase the width of the exported contents by applying the following class to their wrapper (paren) element(s):
.k-pdf-export <parent element selector>
For example if wrapper is a <div> tag with a class .wrapper import this class:
If there are multiple <p> tags, set
You can find more information about the special .k-pdf-export class in the following help article: Change the Exported Content before Export