Most web servers do not deliver PDF files as direct downloads, instead they allow the browser to display them instantly if the browser has this capability. When using the Chrome browser of SiteKiosk Windows this means that most PDF files are shown in the integrated PDF viewer. You can allow downloads in the configuration of SiteKiosk (see below), then a user can click the little download icon in the upper right corner of the PDF viewer to actually download the file, just like in any other modern browser.
Depending on the intended use of the kiosk system, you might want to avoid having your users go through all these steps to download the file or you even want the PDF to automaically open in another application. This can be achieved by manually adding the Chrome command line switch disable-pdf-extension to your SiteKiosk configuration file (assuming it is not possible to change the server behaviour for PDF delivery itself).
Open your SiteKiosk configuration with an editor like Notepad and look for the lines:
Change it to this:
Save the file and open the SiteKiosk configuration editor to use the GUI to make additional changes. Go to Start Page & Browser -> Chrome Browser -> Customize -> Downloads. Enable the download option. Select the PDF file type and click on Edit. Choose Open the following application (or one of the other methods to handle the file) and select the application you want PDF files to be opened with. Finally you might also want to check the Autostart checkbox to automatically start the PDF with the selected application after the download.
Note that SiteKiosk Windows will still show the file in the browser and do the above as additional actions on top of the default behaviour.
Also note that there are additonal command line arguments that can be used for other purposes. See https://devblog.provisio.com/post/2016/09/19/Starting-SiteKiosk-Windows-with-Chrome-Command-Line-Switches.aspx for further information.