You try to email a document and see the message every office worker dreads: the attachment is too large. Large PDFs are a common problem, especially when they contain scanned pages or high-resolution photos. The good news is that you can usually compress a PDF a lot without making it unreadable. This guide explains why PDFs get big and walks through simple ways to shrink them on a computer or phone.
Why are some PDFs so large?
A PDF that contains only text is usually small. File size grows when it includes high-resolution images, scanned pages stored as pictures, embedded fonts or lots of layered graphics. A 20-page scanned document can easily reach tens of megabytes because every page is essentially a photo. Understanding the cause helps you choose the right fix.
Method 1: Use a free online compressor
Many websites let you upload a PDF, choose a compression level and download a smaller file. These are quick and need no installation. However, you are uploading your document to a third-party service, so avoid them for anything confidential such as contracts, ID documents or medical and financial paperwork. Read the service's privacy notice and check how long it keeps files. For sensitive documents, use one of the offline methods below.
Method 2: Use tools you already have
On a Mac
Open the PDF in Preview, choose File > Export, then pick the Quartz Filter drop-down and select Reduce File Size. Save it under a new name. The result can be noticeably blurrier for image-heavy documents, so compare it with the original.
On Windows
Windows has no built-in PDF compressor, but many PDF readers and editors offer a Save as Reduced Size or Optimise option, and some office suites can export to PDF with a minimum-size setting. If you created the PDF from a Word document, try exporting again and choose the standard or minimum size option rather than the high-quality print option.
On a phone
Several mobile PDF apps include a compress option. As with websites, think about privacy before sending sensitive files through an app you do not know.
Method 3: Reduce the source before you create the PDF
The best compression often happens before the PDF exists. Resize large photos before inserting them into a document, because pictures from a modern phone are far bigger than needed for a page. For scans, set the scanner to 150 to 200 dpi for normal text documents instead of 600 dpi. Choose black-and-white mode for text-only pages, which reduces size dramatically. If you regularly take photos for documents, our guide on freeing up space on a Windows laptop may help you manage the files you create.
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Safe compression habits
Always keep the original file and save the compressed copy under a different name. Check the compressed PDF by zooming in on text, signatures, tables and images to make sure everything is still legible. If you need a scan to be searchable, note that some compressors remove or flatten text layers. Aim for the smallest file that still looks good, not the smallest file possible.
When to split instead of compress
If a file is still too big, consider splitting it into two PDFs or sharing it through a cloud storage link instead of an attachment. Many email providers limit attachments to around 20 to 25 MB, though limits vary. Keep your wording clear in the message too; our tips on writing professional business emails can help.
Frequently asked questions
Will compressing a PDF reduce quality?
Usually, yes, to some degree, because compression often lowers the resolution of images. Moderate settings are normally fine for reading on screen.
How small can a PDF get?
It depends on the content. Text-heavy files may shrink a little, while image-heavy files can shrink dramatically.
Is online PDF compression safe?
It is fine for non-sensitive files from reputable services, but avoid uploading confidential documents.
Why did my PDF get bigger after compression?
If it was already optimised, a tool can occasionally add overhead. Keep the smaller of the two versions.
Final thoughts
To compress a PDF well, find out what is making it large, use a trusted tool, and check the result before you send it. A few minutes of preparation saves a lot of frustration later.
