If the PDF is scanned images then you could try using a OCR scanning program to get something amenable for conversion into a format your Kobo can read.
Most OCR programs can create a Word document or an HTML file which could be converted into a format suitable for your Kobo. But its highly likely that the scanned text would be riddled with errors. There are tools to help fix some of those, but they have a fairly steep learning curves. And then you would need to invest time and effort to manually find and fix those they can't find and fix.
Abbyy Fine Print is regarded as the best OCR software i.e. it outputs fewer errors than alternative programs - it costs about the same or more than a
middling tablet 
BR