How to Reduce PDF File Size Online for Free
Large PDF files cause real problems — email servers reject attachments over 10MB, WhatsApp has sharing limits, and uploading documents to portals can time out. Compressing your PDF solves all of this. Here is how to reduce PDF file size online for free, instantly, without installing anything.
Why Are PDF Files So Large?
PDFs grow large for several reasons. High-resolution embedded images are the most common cause — a scanned document or photo-heavy report can easily reach 20–50MB. Other causes include embedded fonts, metadata, revision history, and multiple versions saved on top of each other.
Three Compression Levels Explained
- Screen (smallest file): Optimises images to 72 DPI — sufficient for viewing on screens. File size reduction is maximum, typically 50–80%. Use this for email attachments, WhatsApp sharing, and web uploads.
- eBook (balanced): Optimises images to 150 DPI — good balance between quality and size. Suitable for digital reading on tablets, presentations and general sharing.
- Print (high quality): Optimises at 300 DPI — retains high image quality. Use when you need to print the document or when recipients need to read fine detail.
How Much Can a PDF Be Compressed?
Results vary significantly depending on the PDF content. A document with many large embedded photos might compress from 20MB to 2MB — a 90% reduction. A text-only document might only compress 5–15% because text is already stored efficiently. Scanned documents typically compress the most.
Why Scanned Documents Compress So Dramatically
To understand why scanned PDFs see such large reductions, it helps to know how scanning works. A scanner captures each page as a high-resolution photograph — often at 300 DPI or higher in full colour, even if the original page is plain black-and-white text. At 300 DPI, a single A4 page can require several megabytes just for that one image, and a multi-page document multiplies this quickly. Compression reduces this resolution to something more proportionate to how the document will actually be viewed — 72 DPI is more than sufficient for reading text on a screen, since screens themselves don't display much more detail than that for a page-sized image. The dramatic size reduction comes from this gap between "resolution needed to capture fine print detail when printed at full size" and "resolution needed to read the same text on a screen," which for most documents is a very large gap.
Choosing Between the Three Levels in Practice
For most everyday sharing — email, messaging, uploading to a portal that will be viewed on screen — Screen compression is the right default, since the resulting file remains perfectly readable while being as small as possible. eBook compression makes sense when a document will be read on a tablet or larger screen where someone might zoom in somewhat, or when a document contains some images where moderate detail matters (such as photos in a report) but isn't the primary content. Print compression is worth choosing specifically when you know the recipient needs to print the document and the visual quality of images matters for that printed output — for purely text-based documents, even Print-level compression won't make much difference compared to Screen, since there's little image data to compress either way.
Step-by-Step Guide
- Open the PDF Compressor tool
- Upload your PDF (up to 25MB)
- Select your compression level — Screen for maximum reduction, Print for best quality
- Click Compress PDF
- See the before and after file sizes displayed
- Download your compressed PDF
When Compression Does Not Help Much
If a PDF has already been compressed previously, further compression may yield little improvement. Similarly, text-only PDFs without images are already near their minimum size. In these cases, if you need a smaller file, consider splitting the PDF into sections using our PDF Splitter and sharing only the relevant pages.
Reducing File Size Before vs After Other Edits
If your document needs other changes too — combining with other files, removing pages, adding a watermark — the order matters for getting the best final size. Compressing as early as possible, right after scanning or before other edits, means every subsequent step works with a smaller file, which is generally faster for browser-based tools. The main exception is if you plan to add new pages (from another document) that are themselves large and uncompressed — in that case, compressing the final combined document after merging captures the size reduction across everything at once, rather than needing to compress each piece separately beforehand.
Common File Size Targets
- Email attachment: Keep under 10MB for reliable delivery
- WhatsApp document: Under 100MB (much smaller is better for mobile)
- Online portal upload: Check the portal's limit — commonly 5MB or 10MB
- LinkedIn document post: Under 300MB