How to Reduce PDF File Size: A Complete Guide
8 min read · Updated 2026-06-20
A scanned contract that should be a few hundred kilobytes comes out at 50 MB. A presentation exported to PDF is 20 MB when your email allows only 10 MB attachments. PDF file sizes can balloon unexpectedly, and the reasons are not always obvious. This guide explains what makes PDFs large and gives you practical ways to shrink them.
Why PDFs Get Large
High-Resolution Embedded Images
The most common culprit is embedded images stored at full resolution. When you export a PowerPoint slide deck or a Word document with photos, the images are embedded at print resolution — typically 300 DPI or higher. A single photograph at 300 DPI can be several megabytes, and a presentation with 20 slides full of photos can easily reach 100 MB.
Scanned Documents
Scanning a paper document at 600 DPI produces a very large image file for each page. A 20-page scanned contract at 600 DPI can be 50–100 MB. For most purposes, 150–200 DPI is perfectly readable on screen and prints acceptably for text documents.
Unsubsetted Fonts
PDFs can embed the font files used in the document so they display correctly on any computer. If the full font file is embedded rather than just the characters actually used (a technique called font subsetting), this can add megabytes for each font. A document using five custom fonts without subsetting can be 10+ MB just from font data.
Redundant or Orphaned Data
PDF files accumulate hidden data over their lifetime: previous versions of the document saved as incremental updates, deleted objects that were not garbage-collected, duplicate embedded resources, and extended metadata. A PDF that has been edited and saved many times can contain many layers of accumulated bloat.
The Most Effective Compression Techniques
1. Downsample Embedded Images
For on-screen viewing, 72–96 DPI is sufficient. For printing, 150–200 DPI is adequate for text-heavy documents; 300 DPI only matters for high-quality photo printing. Downsampling images from 300 DPI to 150 DPI reduces image data by 75% (half the width × half the height = one quarter the pixels). This is typically the single largest source of size reduction.
2. Compress Images with Lossy Encoding
After downsampling, applying JPEG compression to embedded images reduces them further. A quality setting of 60–75% for embedded JPEGs in a PDF is almost always indistinguishable from higher quality when the images are viewed at normal reading zoom levels.
3. Subset Fonts
Subsetting embeds only the specific characters used in the document rather than the entire font file. If your document uses only basic Latin characters, subsetting can reduce an embedded font from 400 KB to 20 KB. Most modern PDF exporters do this automatically, but older software or certain workflows may embed full fonts.
4. Linearize and Garbage-Collect
Linearizing a PDF restructures it for fast web viewing and removes accumulated incremental update overhead. Garbage collection removes orphaned objects and other internal waste. These techniques alone can reduce a heavily-edited PDF by 20–40%.
Always keep a copy of the original PDF before compressing, especially for legal or archival documents. Compression is a one-way process — you cannot recover the original image quality after the fact.
Removing Unnecessary Pages
Before compressing, consider whether you actually need all the pages. A 50-page report where you only need pages 3–8 for a specific purpose is much better served by extracting those pages first. Removing 44 pages before compressing the remaining 6 will always produce a smaller file than compressing all 50 pages.
Compress the Result
Email Attachment Size Limits
Common email attachment limits: Gmail 25 MB, Outlook 20 MB, Yahoo 25 MB. For very large PDFs, consider compressing to under 10 MB to stay comfortably within limits. If the compressed PDF is still too large, splitting it into sections using a PDF splitter and sending multiple emails is a reliable fallback.
Frequently asked questions
What is the maximum PDF size I can send by email?
Most major email providers allow attachments up to 20–25 MB. Gmail allows 25 MB, Outlook 20 MB, Yahoo 25 MB. Compressing to under 10 MB leaves a comfortable margin. For files larger than 25 MB, use a file sharing service and send a link instead.
Will compressing a PDF reduce text quality?
No. Text in PDFs is stored as vector data (mathematical descriptions of characters), not as pixels, so it is not affected by image compression. Only embedded images lose quality. The text will remain perfectly sharp at any zoom level.
Why is my scanned PDF so large?
Scanned PDFs store each page as a high-resolution image. A 300 DPI scan of an A4 page is roughly 2480×3508 pixels — about 8.7 megapixels. Multiply that by the number of pages, and the file size adds up quickly. Downsampling to 150 DPI and applying JPEG compression to the embedded images dramatically reduces the size.
Can I compress a password-protected PDF?
Generally no, not without first removing the password. Compression requires modifying the PDF structure, which is blocked by encryption. You need the password to unlock the PDF before it can be compressed.
Is there a limit to how much a PDF can be compressed?
Yes. If a PDF contains mostly text (no images), compression gains are modest — perhaps 10–30%. Most of the easy gains come from downsampling images. Once images are already at low resolution and high compression, further compression yields diminishing returns.