Drop box will actively block you opening the link within an Iframe. Sorry I tested this the other day and forgot to post it.
By the way if you just want a down load link the you use a button that will open a url to the link.on the link you provided change :
Hi Mark,thanks for the Info but I have to display the pdfs within a smaller iframe.I guessed it, that Dropbox does something to make life complicate.No, I think, I will keep the pdf library at another posters server and skip Dropbox.
Dropbox really isn't a great solution for anything other than quick tests and shares.
I'd suggest signing up for an AWS S3 bucket if you actually need to host static assets. Reliable and costs nearly nothing.