PDF file size is rarely something you think about until it's a problem — an email bounces back over an attachment limit, a web form rejects the upload, a file takes an unreasonable amount of time to open on a phone. When that happens, compression is the fix, and it usually takes under a minute.
This guide covers what actually makes PDFs large, how much compression to expect, and how to do it with Portimg's PDF compressor.
Why PDFs get large
The size of a PDF is almost entirely determined by its images. A ten-page document made up of text and simple formatting might be 100–200KB. The same ten pages as a scanned document — each page a high-resolution photograph — can easily reach 10–20MB or more. Multi-page reports with embedded charts, photographs, or graphics follow the same pattern: the images drive the size, not the text.
This is why compression works differently depending on the source material. A PDF made from scanned images at 300 DPI has significant room to compress — the embedded images can be resampled to a lower resolution that's still perfectly readable on screen and when printed at standard sizes. A PDF that's already mostly text with minimal images has much less to compress, and you'll see a smaller reduction.
Understanding this helps set realistic expectations. "Up to 90% reduction" is possible on a PDF of high-resolution scanned pages, but it isn't typical for every document. A well-formatted text-heavy PDF might only compress by 20–30%, which is still worth doing but less dramatic.
Compressing with Portimg
Go to portimg.com/compress-pdf, upload your file, and download the compressed version. The tool processes the file in seconds and shows you the resulting file size before you download so you can assess whether it meets your needs. No account required, and files are deleted from the server after processing.
Before compressing, it's worth keeping a copy of the original. Compression is a lossy process for image-heavy PDFs — it reduces image resolution to achieve a smaller file, and that change is permanent. For most use cases the quality difference is imperceptible, but for documents where print quality matters — high-resolution artwork, photography, technical drawings — compare the output carefully before discarding the original.
Other ways to reduce PDF size
Compression handles the most common case, but a few other approaches are worth knowing for specific situations:
Split before compressing. If you only need part of a large PDF, split out the relevant pages first and compress the smaller file. Sharing 8 compressed pages from a 100-page document is faster and more considerate than sending the entire compressed document and asking the recipient to find what they need.
Use digital text instead of scans where possible. If you have access to the source document — a Word file, a spreadsheet, an original design — export it to PDF directly rather than printing and scanning it. A digitally-created PDF is dramatically smaller than a scanned equivalent and doesn't need compression at all.
Remove unnecessary content before compressing. Pages that were included by mistake, duplicate pages, or cover pages that aren't needed for a particular recipient — removing them before compressing reduces both the final file size and the amount of work the compressor has to do.
When compression isn't enough
Some upload limits are strict enough that even well-compressed PDFs won't get under the threshold — certain government portals cap uploads at 1MB or 2MB, which is difficult to hit with a multi-page scanned document even after compression. In those cases, splitting the document into sections and submitting them separately is usually the practical solution, or converting to a different format if the platform accepts alternatives.
For everyday use — email attachments, Drive links, web form uploads — running a PDF through the compressor before sharing is a small habit that prevents most file size problems before they arise. It takes less time than most email clients take to reject an oversized attachment.