There are multiple potential causes for "fuzzy" images when exporting HTML with in5.
This article will help you diagnose and fix issues related to image quality.
Here are some potential causes of low image quality.
By default, when text is rendered as images, it's rendered as HD at 2x the normal website resolution (144ppi).
However, InDesign treats placed PDFs as images, not text, so it may be rendering PDFs at normal resolution (72ppi).
To fix, change the Image Quality setting (under Advanced in the in5 export dialog) to HD (so that it renders images as HD as well).
If you're using Desktop Scaling (or the Mobile Viewport Zoom) to scale your layout to the browser window, it creates the possibility that your layout may be scaled larger than you originally intended and this can make images blurry.
Quick fix: change the Image Quality setting (under Advanced in the in5 export dialog) to HD.
If you need a quality greater than HD (i.e., 300ppi instead of 144ppi), you can use Object Export settings without InDesign and the corresponding Image Quality setting in the Advanced section of the in5 export dialog.
For more details, see the in-depth article on Controlling Image Quality with in5.
If changing the Image Quality setting isn't enough, your layout may be too small to scale up to larger sizes, and you may need to consider changing the size of your pages within InDesign.
Vector Formats (.eps or .ai)
InDesign (and in5) treats these formats as raster images when exporting, so upping the Image Quality setting (described above) is an appropriate fix if your vector images do not look crisp in the output.