How does PDF compression work in PurePDF?
PurePDF compresses PDFs by re-rendering each page to a canvas element using PDF.js and then re-encoding the canvas as a JPEG image at your chosen quality level. The resulting JPEG images are then packaged into a new PDF document. This raster-based approach works best on image-heavy PDFs. Text-based or vector-heavy PDFs may not reduce in size significantly because their content compresses less efficiently than photographs or scanned images.
Which compression preset should I choose?
The Screen preset uses very low JPEG quality (around 50–60%) and is best for PDFs you only need to read on screen — like sharing a draft or sending via messaging apps. The eBook preset uses moderate quality (around 75%) and is a good all-purpose balance suitable for most sharing scenarios including email. The Print preset preserves higher quality (around 90%) and is suitable for documents you plan to print locally. The Custom preset lets you dial in a specific quality percentage and render scale for precise control over output size and clarity.
Why is my compressed PDF sometimes larger than the original?
This happens when the original PDF already contains highly compressed or optimized content. Vector-only PDFs (pure text and graphics) typically compress very poorly with raster-based approaches because re-rendering them as JPEG images actually adds file size overhead compared to their efficient native vector storage. Similarly, PDFs that were already exported at low quality from another tool may produce a larger file when re-encoded. If your PDF doesn't compress well, try the Custom preset with a very low quality setting, or accept that the file is already near its minimum practical size.
Does compression affect the ability to select or copy text in the output PDF?
Yes. Because PurePDF compresses by re-rendering pages as raster images, the output PDF consists of image layers rather than native text objects. This means text in the compressed PDF will not be selectable, searchable, or copyable in standard PDF readers. If maintaining text selectability is important for your use case — for example, for documents that will be indexed by search engines, archived for compliance, or need to support copy-paste — you should use a different compression method that preserves the document structure, such as a desktop PDF editor with built-in compression tools.
Is there a limit to how large a PDF I can compress?
PurePDF supports files up to 200 MB per upload. The processing time scales with the number of pages and the render scale setting — a 50-page PDF at full resolution may take 30–60 seconds on a typical modern computer. There is no server-side page count limit. If you need to compress a very large file, try using the Custom preset with a lower render scale (0.75×) to reduce memory usage and speed up the process. The render scale reduces the resolution of each page, which can significantly shrink output size for image-heavy PDFs.
Can I compress a password-protected PDF?
No. PurePDF's compressor requires an unlocked PDF to function. If you attempt to compress a password-protected PDF, the PDF.js renderer will not be able to parse the pages and the compression will fail. To compress a protected PDF, first remove the password using PurePDF's Encrypt PDF tool or another utility, then upload the unlocked version to the compressor. After compression, you can re-apply password protection using the Encrypt PDF tool if needed.
How much can I expect the file size to be reduced?
Reduction depends heavily on the type of content in your PDF. Image-heavy PDFs (scanned documents, photo books, presentation slides with background images) typically compress 50–80% with the Screen or eBook preset. Text-heavy or vector-only PDFs (contracts, reports, spreadsheet exports) may compress only 5–20% because the original vector data is already more efficient than raster encoding. PDFs that were already exported at low quality will compress very little or may even increase in size.
Does PurePDF compress PDFs without uploading them to a server?
Yes, absolutely. The entire compression pipeline runs in your browser — PDF.js loads and renders the pages locally, the canvas data is JPEG-encoded in memory, and the new PDF is assembled using pdf-lib, all without any network transmission of your file. PurePDF has no server infrastructure that processes your document. This is a deliberate architectural decision to ensure your PDFs remain private regardless of their content. You can use the tool while offline after the page has loaded.
What is the difference between render scale and quality in the Custom preset?
Quality controls the JPEG compression level — higher quality means fewer compression artifacts and less visual degradation, but a larger output file. Render scale controls the resolution at which each PDF page is rasterized before compression — at 1× each page is rendered at its native 72 DPI, at 2× it is rendered at 144 DPI. Reducing the render scale is a powerful way to reduce output size because it decreases the pixel dimensions of each page, resulting in fundamentally smaller images. For very aggressive compression, reducing both quality and render scale together produces the smallest output.
Will compressed PDFs look good on a smartphone screen?
Yes. The Screen and eBook presets are specifically tuned for on-screen reading, including mobile devices. The Screen preset reduces visual quality enough to cut file size dramatically while remaining clear and readable on modern high-DPI smartphone screens. The eBook preset is slightly higher quality and is virtually indistinguishable from the original on mobile screens for most document types. Only when zoomed in significantly might the JPEG compression artifacts in the compressed version become noticeable compared to the original.