If you've ever tried to email a PDF only to get a "file too large" error, you know the frustration. PDF compression is one of the most common tasks in document workflows - and also one of the most misunderstood.
The good news: you don't have to choose between file size and quality. Done right, a compressed PDF should look identical to the original.
Why PDFs Get So Large
Before compressing, it helps to understand what's making your file big in the first place.
Embedded images are usually the main culprit. A PDF exported from a design tool like InDesign or Illustrator can contain full-resolution images that are far higher quality than you need for screen viewing or printing. A 300 DPI image is great for print; it's overkill for an email attachment.
Embedded fonts add overhead too, especially if the PDF includes many typefaces or subsets. Fonts are necessary for text fidelity but can be optimized.
Uncompressed data streams sometimes survive the export process intact - particularly in PDFs generated by older software.
Hidden layers and metadata can also bloat a file with information that's invisible to readers but stored in the document.
The Three Compression Strategies
1. Image downsampling
This is the most impactful lever. Reducing image resolution from 300 DPI to 150 DPI (or 72 DPI for screen-only documents) can cut file size by 60–80% with minimal visible difference on screen.
Most PDF compression tools let you choose a target DPI. For documents that will only be viewed digitally, 96–150 DPI is more than enough.
2. Lossless vs lossy compression
- Lossless compression (e.g., ZIP, LZW) reduces file size without discarding any data. It's safer but achieves less dramatic size reductions.
- Lossy compression (e.g., JPEG for embedded images) discards some image data to achieve smaller files. The trade-off is usually invisible at moderate compression levels.
For most use cases, a moderate lossy setting gives the best balance.
3. Font and metadata optimization
Subsetting fonts (only embedding the characters actually used in the document) and stripping unnecessary metadata can trim a few extra percent off file size - less dramatic than image compression, but worth doing.
What "Quality" Actually Means
When compression tools advertise "high", "medium", and "low" quality settings, they're usually adjusting JPEG compression quality for embedded images:
- High (80–90% JPEG quality): Minimal visible change, moderate size reduction
- Medium (60–75%): Good balance for most documents
- Low (30–50%): Noticeable in high-contrast images and fine text, suitable only for rough drafts
A setting of "medium" is appropriate for almost all professional use cases - presentations, reports, forms.
When to Compress vs When Not To
Compress when:
- Sending files via email (most servers cap attachments at 10–25 MB)
- Uploading to a web portal or CMS
- Archiving large document sets
- Sharing documents that will only be viewed on screens
Don't compress when:
- Sending to a printer (keep full resolution)
- Archiving legal or regulatory documents where exact fidelity is required
- The original is already small (there's nothing to gain)
Step-by-Step: Compressing with SupaPDF
- Go to Compress PDF
- Upload your file - you can drag and drop or click to browse
- Choose your compression level (we recommend Medium for most documents)
- Click Compress
- Preview the before/after file size and download when satisfied
SupaPDF shows you the exact size reduction before you commit, so you can adjust if needed.
Tips for Better Results
- Start with the original, not a previously compressed file. Re-compressing an already-compressed PDF degrades image quality without meaningful size reduction.
- Check the output at 100% zoom before distributing. Compression artefacts are most visible at full size.
- For scanned documents, OCR text layers can be removed if they're not needed - they add size without visual content.
Compression is one of the simplest wins in document management. A 10 MB report that becomes 1.5 MB is easier to share, faster to upload, and cheaper to store - with no visible difference to anyone who reads it.