PDF File Size: What Makes PDFs Large?

Images are usually the single largest contributor to PDF size. A high-resolution photo embedded at 300 DPI can easily take several megabytes. The same image saved as a low-quality JPEG at 72 DPI might be 5% of that size. PDFs store images in their original encoding — JPEG, PNG, or uncompressed — so an oversized source image becomes an oversized PDF.

The fix is rarely "remove the image." It is to use the right resolution for the intended use (screen vs print) and an appropriate compression format. A photo destined for a website does not need print-quality resolution.

PDFs can embed fonts so the document looks identical on machines that do not have those fonts installed. A full font file can be 200 KB or more, and a document using several fonts can spend most of its size on font data alone.

The standard mitigation is font subsetting: embedding only the glyphs (characters) actually used in the document. If a 200 KB font is used for a single sentence of English text, a subset might be just 10 KB. Most PDF tools subset by default, but not all — and a PDF that embeds full fonts will be noticeably larger than one that does not.

A scanned PDF is essentially a stack of images, one per page. Because each page is a full-page raster image rather than selectable text and vector graphics, scanned PDFs are the largest common variety. A 20-page scanned document can easily reach 20–60 MB.

The only way to substantially shrink a scanned PDF is to re-encode the images at a lower resolution or use a compression format designed for bitonal text (such as JBIG2 or CCITT Group 4). Converting a scan to selectable text through OCR changes the file from images to text and vectors, which is dramatically smaller — but OCR quality varies and is not always available.

PDFs can carry redundant data: the same image placed on every page, multiple copies of a font, or verbose metadata and hidden content layers from the authoring application. Some office exporters also leave unused objects in the file that were part of intermediate drafts.

Re-saving a PDF can drop these unused objects and consolidate duplicates, often with no visible change. This is what acompress operation typically does: it rewrites the file with object streams and discards anything unreferenced.

If your PDF is larger than you expect, the cause is almost always one of the four above. You can shrink it without uploading it anywhere: run it through a localcompress PDF tool that re-saves with object streams and drops unused objects. For image-heavy PDFs, the biggest wins come from re-encoding the source images before they ever enter the PDF.