Why does my PDF look blurry?

Start by zooming in

Open the file and zoom to 400%. What happens to the text tells you which problem you have, and they need opposite fixes.

• Text stays razor sharp, pictures go soft. Normal. The text is vector outlines; the images are pixels. Only the images need attention.
• Text goes soft too. The page is an image — a scan, or a file that was rasterised. Nothing in it is text any more.
• Everything is sharp on screen but prints badly. A resolution problem: enough pixels for a monitor, not enough for paper.

Not enough pixels to begin with

Resolution only means something once you know the size something is printed at. The same image is sharp small and soft large, and the file cannot tell you which it is.

An image taken from a web page is usually around 72 pixels per inch — fine on the screen it was made for, visibly soft on paper. Print wants roughly 300 at final size.

This one cannot be repaired. Detail that was never recorded cannot be added back, and any tool promising to "enhance" it is inventing plausible pixels rather than recovering real ones. The fix is to go back and get a larger original.

It was sharp until I compressed it

Then compression is doing exactly what it was asked to. Shrinking a PDF mostly means reducing image resolution and re-encoding — that is where the bytes are, so that is where they come from.

Two ways it shows up:

• Soft photographs. The images were downsampled. Compress less, or start from the uncompressed original.
• Soft everything, including text. The compression level rasterised the pages. Any tool that does this should say so — on this site only the Maximum level does, and it says so on the page.

Keep the full-quality file. Compress the copy you email, and send the printer the original. How PDF compression works covers what you spend at each level.

What to actually do

1. Find the original. The uncompressed PDF, or the document it was exported from. This solves it outright more often than anything else.
2. Re-export rather than re-compress. Going back to the source and producing a new PDF beats trying to repair the compressed one, every time.
3. Replace the offending image. If one photograph is the problem, swap in a larger version and rebuild the page.
4. If it is a scan, rescan at 300 DPI. OCR will also work far better, for the same reason.
5. Check it is not just the viewer. Some readers render at low quality while scrolling and sharpen a moment later. Stop, wait, then judge.