I think I understand the problem.
You have made a Web document in InDesign and specified the dimensions in pixels (you say it's
1200x5760 px - is it really that narrow?).
Then you place text and images in different resolutions. So while working in InDesign, you might be able to zoom in and see details in a higher resolution than you export to. This could be partly causing the confusion.
Then you export a pdf. But a pdf always has physical dimensions not pixel dimensions! InDesign will assume that you want
72 ppi, so for example a document with the dimensions
72x72 px will create a pdf with the dimensions
When you preview this pdf in Acrobat at 100%, you are not guaranteed that the pixels will be shown at 1:1. You can't even be sure that the preview is 1 inch on the screen!
So Acrobat's 100% is not really trustworthy. And do you even want to see the pdf at 100%? Do you need to able to zoom in on the images? Don't you just want it to fit the presentation screen? You should test how it looks when filling the entire screen.
My guess is that your file is fine - you are just viewing it zoomed.
If you only have one or a few pages, you could just export a png file at the right dimensions.
Let me know if this is any help.