By the sounds of it, your only option is to create a new model for that PDF using v10.5, but there are a couple of things that might help speed things along, depending upon the complexity of the initial model.
First, open the PDF file with v10.5, but don't open the existing model. Have a look at what Monarch has been able to acquire from the PDF file, and then set out to build a new detail template, except that you want to import the detail template (and then any other templates) from the last model. Make any necessary adjustments as things may have shifted a bit here and there.
Then import other elements of the model, such as calculated fields, filters, sort orders, and such, that might have been employed in the last working model.
That ought to get it working more quickly than building it all from scratch.