Update How to embed PDF and other documents

[twMat]

This is IMO a definite improvement but still far from good.

Note; I am also uncertain about some of the claims in the original text so I've left them untouched.

[pmario]

@twMat -- IMO this PR should be created based on the latest http://localhost:8080/#How%20to%20embed%20PDF%20and%20other%20documents tiddler

After a new one is created, this one can be closed.