Why is my PDF so large, and how to make it smaller

"The file exceeds the maximum allowed size" is one of the most frustrating messages when filling in an online form. Many portals limit PDFs to 2, 5 or 10 MB, and a document with a few phone photos can easily go over. Understanding where the size comes from helps you fix it without losing readability.

Where a PDF's size comes from

A text-only PDF created from Word or another program is usually tiny: hundreds of pages can take less than a megabyte. What really adds weight are images. A photo taken with a modern phone has 12 megapixels or more and takes 2 to 5 MB. Convert ten of those and the PDF will easily be 30 MB.

Fixes before creating the PDF

• Lower your camera resolution if you only photograph documents: 8 megapixels is more than enough to read a page.
• Send the photos to yourself through a messaging app: most of them shrink images automatically to a reasonable size.
• Crop the borders that add nothing (the table, the background) with your phone's photo editor.
• Prefer JPG over PNG for photographs: a PNG of a photo can be five times larger.

Fixes for a PDF you already have

• Delete pages you don't need (covers, blank pages, repeated appendices) with Reorder PDF.
• Split the submission: if the portal accepts several files, separate the document into parts and upload them separately.
• Use your system's "Print > Save as PDF" option, which sometimes produces more compact files.

How big should a scanned document be?

As a rule of thumb, a page scanned at good on-screen quality takes 150 to 500 KB. A ten-page document should stay below 5 MB. If your file is far above that, the images almost certainly have more resolution than needed.