Hi Onur,
Hmm, I wasn't aware of that... Like the link states, it could be a good workaround to open the URL to get the pdf in a new tab/window.
window.open("https://server_hostname/path/to/gw/service/pdfEntity/$value")
As I said, I'm not sure the content-disposition will solve the issue you are currently having (I suppose not when reading the link you have provided). Anyway, the content-disposition http header does not have to be handled by the UI. Basically you are telling the browser to show the pdf inline (and not as a download).
Good luck!
Best regards,
Maarten