The Best Way to Compress a PDF: Choosing High, Medium, or Low

By PDFwarp · · 6 min read

PDFwarp's Compress PDF tool gives you three levels before you download. Here's what each one actually does and how to pick correctly.

"Best" depends on what the file is for — a PDF you'll only ever view on a phone screen has a different best-compression answer than one going to a print shop. PDFwarp's Compress PDF tool gives you three levels — High, Medium, Low — and shows the resulting size for each before you commit. This is about how to actually choose between them, not just what compression is.

For the mechanics of how image compression reduces file size (DPI, JPEG quality, the math behind a 15 MB scan becoming a 3 MB file), see How to Compress a PDF Under 1 MB for Email — this post assumes that groundwork and focuses on the decision itself.

The three levels, what they actually trade off

High compression pushes image resolution down to screen-only quality and applies aggressive JPEG re-encoding. You get the smallest possible file, and on a screen it usually looks fine — but zoom in and you'll see it, and it will look visibly soft if printed. This is the right default when the file's entire lifecycle is "email it, someone reads it on a laptop or phone, done."

Medium compression is the one most people should reach for first. It resamples images that are above roughly screen resolution down to a level that holds up both on screen and in a reasonably casual print (a stapled handout, not a glossy brochure). It's the level PDFwarp defaults most users toward because it's the best single guess when you don't know exactly how the file will be used.

Low compression barely touches image quality — it removes duplicate embedded objects (a logo repeated across pages as separate copies instead of one shared resource) and only downsamples images that are far above print resolution, re-encoding them at high quality. Size reduction is modest, usually well under what High or Medium deliver, and the difference isn't visible. This is the level for anything going to print at real quality, or any document where "smaller" matters less than "identical to the original."

A decision framework, not a guess

Ask two questions before picking a level:

1. Will anyone print this at more than casual quality? If yes — a signed contract someone will file physically, a design proof, anything with photography that needs to hold up close — use Low, or don't compress at all if the original is already a reasonable size.

2. Is this going somewhere with a hard size limit, or do you just want it smaller? A hard limit (an upload portal capped at 5 MB, an email attachment ceiling) means pick the lowest level that clears the limit — don't over-compress past what you need to, since every step down costs image quality you can't get back without the original. If there's no hard limit and you just want a leaner file, Medium is the sensible default.

Whichever level you pick, check the result before you send it: zoom in on photos, signatures, and logos, because those show compression first.

Five scenarios, worked through

When a level doesn't do what you expected

Medium barely shrank the file. The PDF probably wasn't image-heavy to begin with — text and vector content aren't touched by compression level at all, so a text-dominant document will show a small percentage change no matter which level you pick — typically 5–15%. That's not a bug; there's nothing there for compression to work on.

The file came back the same size, or slightly larger — actually, it won't. This usually means the source was already compressed. PDFwarp checks for this before handing back a result: if re-compressing wouldn't actually shrink the file, it returns your original file instead of a larger one, so you'll never download a "compressed" copy that's bigger than what you uploaded.

High compression made a specific image look noticeably worse than the rest of the document. That's usually a photograph or scanned signature sitting in an otherwise text-heavy file — the rest of the page held up fine because text isn't affected, but the one image-heavy element took the full brunt of the aggressive setting. Worth checking Medium instead if that specific element matters.

FAQ

Is Medium always the safe default?

For documents you're not sure how will be used, yes — it's the level most likely to look fine both on screen and in a casual printout. For anything with a known print destination or a known hard size limit, use the decision framework above instead of defaulting blindly.

Does compression level affect how long processing takes?

Not really — the three levels differ in image settings (resolution and quality), not in the processing steps involved, so there's no meaningful speed difference between choosing High, Medium, or Low. What actually determines how long a compression takes is your file's size and how much image content it has, not which level you pick.

Can I compress the same file at multiple levels to compare before choosing?

Yes — PDFwarp's Compress PDF checks and shows the resulting size for High, Medium, and Low before you download, so you can compare all three without committing to one and re-uploading.

I need the absolute smallest file possible — is there anything below High?

High is the floor of what the standard compression levels offer. If you're still over a size limit after High, the next lever isn't a more aggressive image setting — it's removing pages, splitting the document, or switching from email attachment to a shared link. See How to Compress a PDF Under 1 MB for Email for that tradeoff.