If a PDF is unexpectedly large, images are almost always the cause. A document with a few full-resolution photos or scans can easily be 20–50MB. Because most PDF editors embed images at whatever resolution you give them, the single most effective way to shrink a PDF is to compress the images before they go in.
Why PDFs get so big
Text and vector graphics in a PDF are tiny. Embedded raster images (photos, scans, screenshots) are not — and if they're inserted at full camera resolution, the PDF carries all those pixels even though the page only needs a fraction of them. Compress and resize the images first and the PDF shrinks proportionally.
The right resolution for PDF images
- On-screen / email PDFs: ~150 DPI is plenty. For a full-page image that's roughly 1240 × 1750px.
- Print PDFs: 300 DPI for crisp print, but only where the image truly fills the page.
- Scans of text documents: grayscale and moderate resolution keep them readable and small.
Workflow: Compress images → insert into Word/Pages/Docs or your PDF tool → export. Doing it in this order avoids the bloat entirely, instead of trying to shrink an already-huge PDF afterward.
Photos vs scans
Save photographs as JPEG at quality 75–85%. For text scans and line art, a reduced-color PNG can be smaller and sharper. Either way, resizing to the page scale does most of the work.
How to shrink a PDF by compressing its images
Compress each image first
Before building or exporting your PDF, drop the photos and scans into the tool and compress them. This is where the size savings come from.
Resize to document scale
A full page at print resolution rarely needs more than ~1500–2000px wide. Resize, then compress at quality 75–85%.
Insert the compressed images and export
Add the smaller images to your document and export the PDF — it'll be a fraction of the original size.
🔧 Try Compress Image — free, private, no upload
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Open Compress Image →Frequently asked questions
How do I reduce the size of a PDF?
Compress the images before adding them to the PDF. Images are what make PDFs large, so resizing and compressing them first shrinks the document dramatically.
What resolution should images in a PDF be?
About 150 DPI for screen/email PDFs (≈1240×1750px for a full page) and 300 DPI for print, only where an image fills the page.
Should PDF photos be JPEG or PNG?
JPEG for photographs; reduced-color PNG for text scans and line art. Resize to page scale either way.
Is my document uploaded?
No. Images are compressed in your browser, which matters for confidential documents.