I have a workflow that generates email messages. It uses a unique ID to attach a PDF file of the same name as the unique ID. I would like it to skip the record if it doesn’t find a matching PDF. Can this be done through workflow?
You’ll have to be more verbose about your question and the details you share if you want help. Start with the premise that we don’t know anything about your environment, data, plugins you use, etc… 
Most likely it is feasible but to guide you properly, we need to know more…
The workflow does have a simple file condition check to see if a file exists. You could always put your mailing logic behind this condition.