PDF Viewer visual not working with base64

Thanks Anonymous for your answer,

what you say is already clear to me. I know the visual requires column storing base64 as argument and I need to select one document at a time, but my issue comes before. As you can see in the Advanced Editor query I pasted in red in my post, documents are already converted in base64 when I come in Power BI, as this step is accomplished by my web service. In the sample report you can find at the link I posted, base64 strings are already parted in 2 different columns and then they are concatenated to get the argument column, but I get my string which is larger than 30.000 characters and it's the max you can store in tabular models, as PDF Viewer visual documentation says.

I use a web service because I need to receive and convert documents from another web service. Even if I use your Python query, where should I write that? 

Anyway, how can I fix that?

Nick