How to Reduce PDF Size Online for Free
Practical methods to reduce PDF file size without losing quality — covering image optimization, compression tools, and browser-based solutions that work on any device.
PDF files grow quickly. A presentation becomes 40 MB after adding images. A scanned document is enormous because each page is a raw photo. Email attachments bounce. Upload forms reject your file. The solution is PDF size reduction — and you don't need expensive software to do it.
Why PDF Files Get So Large
Understanding what makes PDFs large helps you choose the right reduction method. Most large PDFs fall into one of these categories:
- High-resolution embedded images — the most common cause of large PDFs
- Scanned documents — each page stored as an uncompressed or lightly compressed photo
- Embedded fonts — full font files rather than subsets
- Embedded metadata, thumbnails, and document history
- Unoptimized export settings from software like Word or PowerPoint
Method 1: Compress Images Before Creating the PDF
If you're creating a PDF from images (photos, scans, graphics), the most effective approach is to compress the images before combining them into a PDF. A JPEG at 80% quality is visually identical to the original but 60–70% smaller — and that saving carries directly into the PDF.
- Use Convertly Tools' Compress Image tool to compress each image before conversion.
- Target 75–85% JPEG quality for photographs — invisible loss at screen resolution.
- Resize images to the maximum display size needed — a 300 DPI image for screen use only wastes 4× the pixels needed.
- Use Convertly Tools' Image to PDF tool to combine the compressed images into a PDF.
For a PDF meant for screen viewing only (email, downloads), resize images to 1200px wide maximum before converting. This alone can reduce file size by 70–80%.
Method 2: Export PDF with Lower Image Quality Settings
If you're exporting a PDF from Word, PowerPoint, or a design tool, look for PDF compression or quality settings in the export dialog. Most tools offer a 'minimum size' or 'screen quality' option that compresses embedded images automatically.
- Microsoft Word: File → Save As → More options → Optimize for: Minimum size
- Google Slides: File → Download → PDF (uses web-optimized compression)
- Adobe Acrobat: File → Reduce File Size → or → Save as Optimized PDF
- macOS Preview: File → Export as PDF → Quartz Filter: Reduce File Size
Method 3: Reduce Image Resolution Before PDF Creation
Photographs from modern cameras are often 4000–8000 pixels wide. A PDF for screen display only needs images to be 1200–1920 pixels wide. Resizing to the actual display dimensions removes 80–90% of the pixels — and most of the file size.
- Open Convertly Tools' Resize Image tool.
- Upload your images and resize to 1200px width (maintaining aspect ratio).
- Download the resized images and convert to PDF using the Image to PDF tool.
Method 4: Use PDF Compression Software
For PDFs that already exist and need to be reduced (reports, scanned contracts, presentations), online PDF compressors can help. These tools re-encode embedded images at lower quality settings without changing the text or layout.
What Level of Compression is Safe?
| Use Case | Target Quality | Image Resolution | Expected Size |
|---|---|---|---|
| Email attachment | 60–70% | 72–96 DPI | Very small |
| Web download | 75–80% | 96–150 DPI | Small |
| Professional review | 85–90% | 150 DPI | Medium |
| Print preparation | 90–95% | 300 DPI | Large |
| Archival | 95–100% | 300+ DPI | Very large |
Use Convertly Tools to compress images first, then combine into a small PDF — free, private, browser-based.
Compress & Convert Images to PDF