Blurry pdf

A few things here to consider. If you are exporting to High Quality and haven't changed the settings within that preset, by default, the images are being downsampled to 300 ppi if they are over 450 ppi, ... 300 is more than enough for print. If your original image was well above that theshold, say 500 ppi, then in the process of downsampling to 300 it involves taking averages of some adjacent pixels to create new ones... this may cause it to be a bit blurry compared to the original when you zoom in on it, but makes little difference, if at all, in print.

As for text, if it's still vector, it shouldn't be affected. However, if it's rasterized or has an effect/transparency that will cause it to be rasterized, you'll find that 300 ppi is not at all enough, and with anti-aliasing, will make text blurry especially the smaller it gets. 

Also: where are you viewing the PDF? Does it look the same if you use another viewer?

Can you show a sample of before and after?

Are you viewing the PDF in Acrobat?

What are those objects? Is any effect applied? Can you show a screenshot?

After exporting a design from illustrator to pdf, some of the text and images look blurry when opened. Even with high quality print settings. What'd be causing this? How to keep it sharp?

@serenity_2796 this is the Photoshop forum, try posting on the Illustrator forum instead