Compress PDF to Meet Email Attachment Size Limits: Re-rendering Compression Guide

For most PDF files, more than 90% of the total size comes from embedded high-resolution images, while text content and metadata take up less than 10% of the space. PDFs generated by scanning are essentially stitched together from page-by-page images, so their size grows rapidly as the scanning resolution increases.

If a native PDF inserts a large number of uncompressed photographed photos or design exported images, it may also exceed the email attachment size limit. Plain text PDFs are usually small in size and rarely reach the 20-25MB limit threshold.

This compression method re-renders each page of the PDF as a bitmap with JPEG compression, then stitches the pages back into a new PDF. This mechanism can achieve a 50% to 90% size reduction, and most PDFs that exceed the limit can be compressed to the target range.

The compression ratio is determined by the final image resolution density and JPEG compression quality. The lower the density and the higher the compression level, the smaller the final PDF size, and the corresponding visual clarity will decrease accordingly.

Current PDF re-rendering compression tools provide three fixed quality settings, which correspond to three resolution densities: 73DPI, 100DPI, and 150DPI. Users can select the corresponding setting according to their usage scenario to balance file size and clarity.

For a PDF of around 20MB with a large number of images, selecting the 73DPI setting can compress it to less than 3MB, and selecting the 150DPI setting can usually compress it to less than 10MB, both of which meet the attachment size requirements of most email services.

The compression method of re-rendering entire pages to bitmaps has clear tradeoffs. Compressed PDFs lose the original text layer: the generated PDF cannot be text-searched, and text content cannot be copied or extracted from it.

Plain text PDFs are already small in size. Re-rendering compression converts text to bitmaps, which will instead increase the file size and brings no compression benefit. For PDFs that have already been compressed via re-rendering, additional compression will hardly reduce the size, and may even result in a larger file in some cases.

The PDF compression tool supports PDFs with no more than 50 pages for processing in the asynchronous queue. PDFs exceeding the page limit cannot be submitted for compression. Users can download the processed file via the result link after processing is complete.