Compress a PDF Without Losing Quality: What DPI, JPEG Quality and Downsampling Actually Do
The situation
Every compressor promises "no quality loss". Then the 120 MB portfolio becomes 8 MB and the renders look like they were faxed. Or the opposite: the "lossless" option runs and the file drops from 120 MB to 117 MB.
Neither tool lied, exactly. They used the word quality to mean different things. This guide explains what is actually being changed so you can choose settings on purpose.
Two kinds of compression inside a PDF
Lossless rewrites the file without changing a single pixel or glyph:
- deflating uncompressed streams,
- grouping objects into object streams (PDF 1.5+),
- subsetting fonts to the characters used,
- removing duplicate images, unused objects, and the leftover revisions from repeated saves.
Typical saving: 5–30%. On a scan or a photo-heavy export: close to nothing, because the images were already JPEG.
Lossy changes the images:
- downsampling — reducing pixel count to a target resolution,
- re-encoding — JPEG at a chosen quality level,
- color conversion — color to grayscale, or dropping a color profile.
Typical saving: 50–95%. This is where the big numbers come from, and this is what "without losing quality" refers to: lossy compression tuned so that you will not notice the loss at normal viewing. Whether you notice depends on what you look at and how closely.
How much resolution you actually need
Resolution in a PDF is about pixels per inch at the size the image is placed on the page. A 3000-pixel-wide photo placed 3 inches wide is 1000 ppi. Downsampling it to 200 ppi throws away 96% of its pixels and changes nothing visible on the page, on screen or in print.
| Use | Image resolution that is enough |
|---|---|
| Reading on screen (laptop, tablet) | 96–150 ppi |
| General office printing | 150–200 ppi |
| Small print, footnotes, handwriting, stamps | 200 ppi minimum |
| OCR input, archival of text pages | 300 ppi |
| Fine line art, maps, engineering detail | 300–600 ppi, or keep it vector |
Printers put dots at 600–1200 dpi, but a photograph needs far fewer pixels than the printer has dots; 200–300 ppi is the long-standing print standard for images.
JPEG quality
JPEG quality is a dial, usually 0–100. Roughly:
- 85–95: visually identical to the original for photos; modest savings.
- 70–85: the standard range; fine for photos, renders, scanned documents.
- 50–70: visible softening and faint blocking around edges; acceptable for reference copies.
- Below 50: artifacts on text and lines; avoid for documents.
JPEG is designed for photographs. Line art, diagrams and text screenshots develop halos even at high quality settings; keep those as lossless (Flate/PNG-style) images or, better, as vector content.
Color to grayscale
A color image has three channels; grayscale has one. Converting typically cuts an image's size by 40–60% and is invisible on documents that were black-and-white to begin with — which includes most scanned contracts, forms and letters. It is the single largest "free" saving on a scan that does not have colored stamps or highlights that matter.
Vector content is never downsampled
Text, lines, shapes and fonts are instructions, not pixels. Compression settings do not touch them; a drawing exported from CAD stays sharp at any zoom after compression. If a tool blurs your text, it rasterized the page — that is a different (and bad) operation, not compression.
How to judge the result
- Open the original and the compressed file side by side.
- Zoom both to 100%, then 200%, on the page with the smallest text and on one photograph.
- Read a number. Numbers are where errors cost money.
- If it will be printed, print one page.
- If it had an OCR layer, search for a word.
If it passes all five, the quality loss is real but irrelevant. That is the correct target.
Choosing settings on purpose
- Review copy for screen: 120–150 ppi, JPEG 75, grayscale if the content allows. Expect 70–90% reduction on scans and photo-heavy files.
- Formal filing with small print: 200 ppi, JPEG 80, grayscale for scanned pages. Expect 50–80%.
- Archive or master copy: lossless only. Expect 5–30%. Keep the original anyway.
A target-size tool such as PDF300 runs this trade-off backwards: given the size you must hit, it finds the mildest settings that get there, which is usually a better outcome than guessing a preset and hoping. Either way, the five-step check above is what tells you whether the result is acceptable.
FAQ
Is compressing a PDF twice worse than once?
Each JPEG re-encode loses a little more; the loss compounds. Two passes at quality 80 are worse than one pass at quality 70. Always compress from the original, never from a previous compression.
Why does the "lossless" option barely shrink my scan?
Because a scan is already a JPEG. Lossless methods can only tidy the container around it. Real savings on a scan come from downsampling and grayscale.
Will my drawings or charts get blurry?
Not if they are vector — compression does not touch them. Raster backgrounds (aerials, scanned base maps) do get downsampled; keep those at 150–200 ppi for review sets and confirm requirements for sealed deliverables.