How to compress a PDF: make files smaller without wrecking them
By Nasir Uddin Shamim4 guidesCompress a PDF by choosing between two methods. Lossless structural compression subsets fonts and removes unused objects, typically saving 10-30 percent with zero visible change. Image downsampling re-encodes embedded photos at a lower DPI, which can cut a scanned document by 80-95 percent but is not reversible.
A PDF gets big for one of two reasons: it contains large images, or it contains a lot of accumulated structural junk. The fix is different for each. Structural compression rewrites the file, subsets fonts, drops orphaned objects, packs the cross-reference table, and never touches a pixel. Image compression downsamples photos and scans to a target DPI, which is where the real savings live. These guides cover both, plus the email limits that started the problem.
Every guide in this topic
How to compress a PDF file for free
To compress a PDF for free, open it in a browser-based compressor and pick one of two methods. Lossless structural compression subsets fonts and strips unused objects without touching a pixel. Image downsampling re-encodes photos at a lower DPI. Text documents shrink 10-30 percent; scans can drop by 90.
How to reduce a PDF file size for email
Compress the PDF to a 150 DPI target first, which is enough for on-screen reading and usually gets a scan under 5 MB. Gmail and Yahoo cap attachments at 25 MB, Outlook.com and iCloud Mail at 20 MB, and MIME encoding adds about 33 percent, so aim below 18 MB.
How to compress a PDF without losing quality
Truly lossless PDF compression means font subsetting, object streams and removing unused objects, it changes nothing visible and saves 10-30 percent. Anything beyond that downsamples images. For documents you will only read on screen, 150 DPI is visually indistinguishable from 300 and saves about 75 percent.
Why is my PDF file so large? 8 reasons
A PDF is usually large for one of eight reasons: it is a scan stored as images, images are embedded at full resolution, fonts are embedded whole instead of subset, resources are duplicated per page, saves were incremental, images are cropped only visually, forms and attachments are attached, or colour profiles and transparency are heavy.
Tools for this job
Compress PDF
Shrink file size
Split PDF
Break into ranges or pages
PDF to JPG
Pages → images
Repair PDF
Fix damaged files
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