Why text can look blurry
Real PDF text can remain sharp, but in a scanned PDF the letters are part of an image. Strong image compression can therefore soften small text as well.
Check the real viewing or printing condition
A file that looks fine on a monitor may show problems in small printed text, QR codes or fine lines. Judge quality in the way the document will actually be used.
Inspect the difficult pages, not only page one
Check small notes, thin table lines, QR or barcodes, dark gradients, signatures and stamps. These areas reveal excessive compression quickly.
Keep the original file
Compression can include irreversible quality changes. Save the original and create a separate sharing or submission copy so you can adjust again if needed.