At this time, being able to open a purchase order by job report isn't available. Being able to run a report by job is currently accessible for customer related transactions.
To get the information that you need, I recommend exporting the Open Purchase Order Detail report to Excel and manually customize it by job.
I can see the importance of having this option for your business. Please know that our developers are always finding ways to improve the product. I'll take not and personally submit this preference this to them so they can put it into consideration.
I also encourage you to visit the QuickBooks Online Blog to be updated with the latest news and product enhancements.
Here are some articles that you can check out about the reports that are available on your account and on how to customize it: