Whether or not you can take a linear test on a mobile device is dependent on whether or not your instructor has opted to require Platinum Secure Testing for your test.
If they have not activated Platinum Secure Testing for your test you should be able to use a browser on your smartphone or tablet to take your test.
If they have activated Platinum Secure Testing, you will not be able to take the test using a mobile device, unless you have an iPad and download the Secure Testing App.
To see if your instructor is requiring Platinum Secure Testing you would want to check your "Testing Instructions" Page.
When you click on the "Take Test" button from your student homepage inside EMSTesting or Platinum Tests or click on the "Open" button from your My.platinumed.com Dashboard, you will be taken to the "Testing Instructions" page prior to entering your test.
Warning: Do not click "Begin Test" unless you are actually ready to begin your test at this time!
On this instructions page if you see a section with the heading "Secure Testing Environment Required" this means you will have to download the additional software to the device you will be testing on or download the App for your iPad to begin taking your test.