A blurry PDF has one of a small number of specific causes, and — this is the part that trips people up — only some of them can be fixed after the file already exists. Knowing which one you have tells you whether there’s anything to do about it.
By far the most common cause is a low-resolution scan. A scan’s sharpness is set once, at capture, by its DPI (dots per inch) — a scanner or scanning app set to 100–150 DPI produces soft, slightly fuzzy text even before the file goes anywhere, and below about 150 DPI small text especially starts to look genuinely blurry rather than just plain. Zoom in a long way to tell: a low-DPI scan gets progressively softer and more pixelated the further you zoom, rather than staying sharp. If you still have the paper, rescan at 300 DPI — see Best Scanner Settings for OCR for the specific settings that produce a sharp, OCR-ready page. If you only have the already-blurry file, resolution that was never captured can’t be recovered; sharpening filters improve contrast, not detail that isn’t there.
Heavy compression is the second common cause. Compression shrinks a PDF mainly by re-encoding its images at a lower resolution or with more aggressive JPEG compression — done well, this is invisible, but done aggressively, the kind an old “reduce to 100 KB” tool applies without asking, it produces visible blockiness and softness, particularly around text edges and fine detail in photos. The tell is a file that looks noticeably worse than a version you remember, or worse than the same document opened somewhere else — blocky squares around high-contrast edges are the signature of over-aggressive JPEG compression specifically. If you have the original, uncompressed file, start from that instead and use Compress PDF, which uses a balanced setting that shrinks file size while keeping text sharp and photos clean, rather than compressing as hard as possible regardless of how it looks.
A third, less obvious cause is unembedded fonts. Text that looks slightly soft or subtly wrong — not photographically blurry, but not crisp either — can be a font substitution rather than a resolution problem: if a PDF doesn’t embed the fonts it uses, the viewer swaps in something close on machines that don’t have the original font, and the replacement can render with different anti-aliasing or slightly different weight, reading as “blurry” even though technically nothing is low-resolution. The tell is that the same file looks sharp on one device and soft on another — a genuine resolution problem looks the same everywhere. The fix is re-exporting the source document with fonts embedded, if you have it; most Word and PowerPoint exports to PDF, including Word to PDF here, embed fonts by default.
Sometimes it’s the viewer, not the file. Some PDF viewers render at a lower quality while a page is loading or zooming, then sharpen once rendering settles — a browser’s built-in PDF viewer is the usual culprit. If a page looks sharp after you stop scrolling and wait a moment, it’s a renderer quirk, not a file problem, and it doesn’t affect printing or how the file looks in a different reader.
When blur genuinely can’t be fixed: if text or an image was captured at low resolution, no downstream tool restores detail that was never recorded — this includes the file itself, once it exists. The only real fix is going back to the source, rescanning the physical document, or getting a higher-resolution export from whoever produced the original digital file.
Converting the PDF to images rather than viewing it? Why Does My PDF Look Blurry After Converting It to JPG? covers that separate cause, which is about export resolution rather than the file itself.
If compression is the cause, run your best available copy through the free Compress PDF tool — no registration wall, no watermark, file gone right after processing.