Removing blurred mathematical expressions into a PDF file and make it vectorized when zooming

Question

I have converted an EPUB file to a PDF. Here the result of zoomed capture. :

zoom of the PDF

As you can see, the maths symbols \sum symbol and delta Kronecker are blurred.

Is there a way to fix this ? I tried to convert the PDF into PS and return to PDF ( pdf2ps and ps2pdf ) but it doesn't work.

Any suggestions ?