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iframe with pdf not responsive

I am setting up a website to display historical documents stored as pdf files on the website and loaded using an iframe with src. On all the browsers I have tried, with the exception of Firefox, if I try to reduce the size of the window, the texts get a horizontal slider and cannot be read properly without moving it back and forth, which is not a responsive behaviour. I gather that the reason that Firefox works differently is that it uses pdf.js. I have tried to use this in conjunction with Framework 6, but all I get is 404 errors. 

foundation 6iframe

I am setting up a website to display historical documents stored as pdf files on the website and loaded using an iframe with src. On all the browsers I have tried, with the exception of Firefox, if I try to reduce the size of the window, the texts get a horizontal slider and cannot be read properly without moving it back and forth, which is not a responsive behaviour. I gather that the reason that Firefox works differently is that it uses pdf.js. I have tried to use this in conjunction with Framework 6, but all I get is 404 errors.