You can select a PDF in Windows or MacOS and drag it to the K2 client window. K2 will then attempt to process the file with the workflow you dropped it on.
There are two ways to do this.
The first way is to drag a job onto the workflow diagram, at the top. Click and hold on a PDF; drag it over to the workflow diagram; let go of the mouse button (dropping the PDF onto the workflow)
The second way is to drag the job to any workflow in the workflow list on the left side of the client window.
You can also drag and drop multiple files.
Here you'll see 6 files get input. Preflight is run on each file and a report is generated. Then the jobs pause at the first pause, after Preflight. It's pretty fast.
Here are 2 youtube clips showing the drag&drop process in slightly greater detail: