"Attachment too large" is one of the most annoying messages in email. Many providers limit attachments to around 20 to 25 MB, and some company systems allow far less. The good news is that most oversized PDFs can be made much smaller without any visible loss of quality.
Why PDFs get so big
The size of a PDF is mostly about images. A page of text takes very little space, but photos and scans can take a lot, especially when they were captured at a much higher resolution than anyone will ever view them. Common culprits:
- Scanned documents saved at high resolution.
- Phone photos turned into PDFs at full camera resolution.
- Presentations and brochures exported with uncompressed images.
- Merged files that contain the same logo or background image many times.
How compression works in Unique PDF
Compress PDF focuses on the images, where the savings are. It resizes pictures that are far sharper than they need to be for on-screen viewing and printing, re-encodes them efficiently, merges identical images that appear more than once, and cleans up unused data. Text and line drawings are left as they are, so the result stays sharp and searchable.
You choose how far to go:
- Light — best quality. Only oversized images are reduced.
- Balanced — recommended for most files. A good mix of size and quality.
- Strong — the smallest size while keeping the document's text, with images at a lower resolution.
For the smallest possible file there is also a maximum compression option that turns each page into an image. It is useful for scans, but the text can no longer be selected or searched, so Unique PDF shows a clear warning before you use it.
Step by step
- Open Compress PDF from the Unique PDF quick menu or the Convert section.
- Add your PDF.
- Choose a level. Start with Balanced.
- Click Compress PDF. You will see the size before and after, and how much you saved.
- Download the smaller file, or go back and try another level.
Tips for the best results
- Compress after merging, not before. Merging first lets Unique PDF remove duplicate images across the whole document.
- Try Balanced before Strong. For many documents the difference in size is small but the quality difference is noticeable.
- Text-only documents are already small. If your PDF has no images, compression will not shrink it much, and that is expected.
- Keep the original. Compression always creates a new file, so you can compare and choose.
Private by design
Compressing a PDF does not require uploading it. Unique PDF processes the file in your browser on your own computer, so confidential documents stay confidential, and there are no limits on file size or the number of files beyond your computer's memory.
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