Why the PNG page looks blurry
If the result looks softer than in a PDF viewer, it almost always comes down to one of the following causes, and only some can be solved during conversion.
1. The scan is the ceiling
If the PDF consists of scanned pages, it only contains the pixels of that scan. A 300 dpi export enlarges that grid instead of adding detail: the blur is already in the source.
2. Output resolution is too low
At 72 dpi an A4 page is only about 595 pixels wide. If the image is shown large on a screen or printed, the browser or printer has to interpolate, which looks soft.
For viewing at original size and for print, 150 or 300 dpi is the better choice.
3. Scaling in the viewer
Image programs often smooth an image when magnifying beyond 100 percent. PDF viewers do not show the same effect because they redraw text. Always judge the result at 100 percent zoom.
4. Text becomes pixels
Inside a PDF, text is stored as vectors and stays sharp at any zoom level. In a PNG it is a fixed number of pixels: the lower the resolution, the softer the edges. That is not a defect, it is the nature of a raster image.
What actually helps
Check the source scan quality first, then choose a higher output level and evaluate the result at 100 percent zoom. If the original is a soft scan, the only real fix is to scan the document again at higher quality.