Large PDF files are frustrating — they bounce back from email inboxes, fail to upload to web forms, and slow down sharing. PDF compression reduces file size by optimizing how the document's content is stored, often achieving 60–90% reduction for scanned documents and 20–50% for text-heavy PDFs. This guide explains the techniques and shows you how to compress PDFs for free without sending them to a server.

Why PDFs get large in the first place

PDF file size grows from several sources:

Embedded images — a scanned document saved as a PDF can easily be 5–20MB per page if the scanner saves at 300 DPI without compression.
Unoptimized fonts — PDFs embed font data. Multiple fonts, or font subsets that weren't stripped, add kilobytes.
Duplicate data — some PDF generators include the same embedded image or resource multiple times.
Hidden layers — unused form fields, annotation layers, or metadata add invisible bulk.
High-resolution images — printing-quality images (300 DPI+) are far larger than necessary for screen viewing.

How PDF compression works

PDF compressors reduce file size through several techniques:

Image downsampling — reducing embedded image resolution from 300 DPI to 150 or 72 DPI, which is sufficient for screen use.
Re-encoding images — converting embedded images to more efficient formats (JPEG for photos, lossless methods for text/graphics).
Removing redundant data — stripping unused metadata, annotations, form data, and duplicate resources.
Stream compression — applying Flate (ZIP-like) compression to data streams within the PDF that weren't already compressed.

How to compress a PDF for free

Picovert's PDF compressor processes your PDF entirely in your browser using WebAssembly — no file upload, no server-side access to your documents.

Open the PDF compressor.
Drop your PDF file or click to browse. Maximum supported size depends on your device RAM; most files under 50MB work well.
The tool automatically compresses the PDF using lossless stream compression and image optimization.
Preview the file size reduction, then download your compressed PDF.

How much compression can I expect?

Scanned documents (high-DPI images) — 60–90% reduction. A 20MB scan can compress to 2–4MB.
PDFs with photos — 30–60% reduction depending on the original image quality.
Text-heavy PDFs — 10–30% reduction. These are often already well compressed by the PDF creator.
Already-compressed PDFs — near 0% reduction. You can't significantly compress a file that's already been optimized.

When does PDF compression make sense?

Email attachments — most email services have a 10–25MB attachment limit. Compress large PDFs before sending.
Web form uploads — government portals, university portals, and HR systems often cap uploads at 5–10MB.
Document sharing — smaller files download faster and are easier to share via messaging apps and cloud storage.
Storage savings — compressing an archive of scanned invoices or contracts can save gigabytes over time.

Privacy: what happens to my PDF content?

Picovert compresses PDFs entirely in your browser using a WebAssembly-compiled PDF library. Your document content — text, images, signatures, financial data — never leaves your device. This is particularly important for contracts, medical records, and financial documents that you wouldn't want processed on a third-party server.

What if I need to combine images into a PDF first?

If you have multiple images (JPG, PNG, WebP) that you want to convert to a single PDF before compressing, use the Image to PDF converter — it combines multiple images into one PDF in your browser, then you can compress the output.