SPOnline IRM PDF protection

Hi all.

I've recently enabled the RMS system in Tenant administration center and in SP Central Admin too, no issues about it. Same for activating IRM on a DL.

The problem is when i must use PDF files: the file I want to upload is aobut 55kb. But when i try to download it, file becomes about 190KB and when I open it i see this message:

 

I tried opening it with all the suggested readers in the link (Nitro, Foxit, AIP client) but with no luck.

How can achieve it? 

No issue with Office files.

Addictional info: my scenario is based on a SP site where external users (es: gmail and liveid account) will access to read and download (seems impossible to prevent downloading, even with readonly permission).

 

Any suggestions?

Thanks a lot, 

Michele