PDF Compressor
Compress PDF files in your browser — adjust DPI and JPG quality to reduce file size quickly and privately.
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Frequently Asked Questions — PDF Compressor
Answers to common questions about compressing PDFs, reducing PDF size, client-side PDF compression, DPI, quality settings, and privacy.
How does this PDF compressor reduce file size?
This tool performs client-side PDF compression by rasterizing each page to an image (canvas) and re-encoding the page images as JPEG at the chosen quality and DPI. That reduces size for scanned or image-heavy PDFs by downsampling and applying lossy JPEG compression. The process uses PDF.js to render pages and jsPDF to rebuild a new PDF from the compressed images.
Will compressing the PDF in the browser upload my file to a server?
No. Compression runs entirely in your browser (client-side) — the file does not get uploaded to any external server during compression. The generated compressed PDF is created locally and offered as a download, so your document remains private unless you choose to send it elsewhere.
Will compressed PDF keep selectable/searchable text?
No — because this tool rasterizes pages to images, text in the resulting PDF becomes part of an image and is no longer selectable, searchable, or copyable. If you need to preserve searchable vector text, consider server-side tools that optimize fonts and remove unused objects (e.g., Ghostscript, qpdf) which can reduce size while keeping text as text.
Which settings best reduce file size: DPI or Quality?
Both matter. DPI controls pixel density (render resolution) — lowering DPI (e.g. 96–150) reduces file size dramatically by producing smaller images. Quality controls JPEG compression level — lower quality (e.g. 30–60) increases compression but can introduce visible artifacts. For scanned PDFs, start with DPI 96–150 and quality 40–60 for strong reductions. For digital PDFs with text, rasterizing may cause loss of clarity — try lower DPI first and verify with the Estimate function.
Why does estimated size sometimes differ from the downloaded file?
We provide two ways to estimate size: a fast sampled estimate and an exact estimate that runs the full compression pipeline. The fast estimate extrapolates from 1–2 sample pages and may be off for mixed content PDFs. The exact estimate runs the complete compression pipeline and sets the exact downloadable blob — this guarantees the estimate equals the downloaded file.
How long does client-side compression take?
Time depends on PDF page count, DPI, CPU, and memory. Small PDFs (1–5 pages) typically finish in seconds. Larger PDFs (tens of pages) at higher DPI take longer and use more memory. Use lower DPI/quality for faster results or use server-side compression for large, production workloads.
Is there a page-count or file-size limit for client-side compression?
Browser memory limits and performance vary. Very large PDFs (many pages or multi-megabyte images) may cause the browser to run out of memory or become slow. We enforce a practical client-side limit on file size (configurable in the script) and recommend server-side processing for very large or mission-critical compression tasks.
What types of PDFs compress best with image-based compression?
Scanned documents and image-heavy PDFs benefit most because compression reduces the size of embedded page images. Digital PDFs that are mostly vector graphics or text may not compress as well with rasterization (and may lose the selectable text). For vector/text PDFs, consider tools that do font/subset optimization or object removal without rasterizing.
Can I preview the compressed quality before compressing the entire file?
Yes — use the Estimate Size control (which runs the same compression pipeline). After the estimate completes, the exact compressed blob is available for download so you can review image quality and adjust DPI/quality before committing to final compression.
Which libraries power this client-side PDF compressor?
This tool uses PDF.js to render PDF pages to HTML canvas and jsPDF to reassemble those canvases into a new PDF file. Both libraries run in the browser and are loaded dynamically to keep the initial page light.
Does compressing the PDF affect image quality for printing?
Yes — lowering DPI or JPEG quality will reduce image fidelity. For print-quality output, use higher DPI (300) and quality (80–100). For on-screen viewing or email sharing, lower DPI (96–150) and quality (30–60) often produce acceptable results with much smaller files.
Is the compressed PDF compatible across PDF readers?
Yes — the output is a standard PDF containing JPEG images as page content and is viewable in modern PDF readers like Adobe Acrobat, browser PDF viewers, Preview (macOS), and mobile PDF apps.
Can I automate or batch compress multiple PDFs?
This client-side tool is focused on single-file compression in the browser. For batch processing, consider a server-side pipeline using Ghostscript, qpdf, or other command-line tools and an API or script to iterate over files.
What if I need to keep text searchable and still reduce size?
If searchable/selectable text is required, avoid rasterization. Use server-side tools that optimize fonts, remove embedded thumbnails, compress streams, and subset fonts. These approaches reduce size while preserving vector text and searchability.
How do DPI and page dimensions relate?
PDF pages are measured in points (72 points = 1 inch). DPI controls how many pixels we render per inch when rasterizing; higher DPI produces larger canvases (more pixels) and therefore larger image sizes. Use the DPI control to balance readability and size: lower DPI = fewer pixels = smaller file size.
Is my data safe — do you keep or log my files?
For the client-side compression path, files never leave your browser. If you use any optional server-side feature (not provided by default here), check the privacy policy and server logs. The tool's client-side workflow ensures privacy and local control.