How to reduce PDF file size
A PDF that is too large to email or upload is one of the most common document headaches. The good news is that most oversized PDFs are big for a single, fixable reason — and once you know what it is, you can usually cut the file dramatically without any visible loss.
Why your PDF is large in the first place
Text is tiny. A hundred pages of pure text is often well under a megabyte. When a PDF balloons to 10, 20 or 50 MB, the weight is almost always images — full-page scans, photographs, or high-resolution screenshots embedded at far more detail than a screen or printer can actually show.
The second culprit is scanning. A scanner saves each page as a picture, so a 'document' that is really a stack of page-images carries all the bulk of photos even though it looks like plain text. That is why scanned PDFs compress so well: there is a lot of redundant image data to trim.
How much can you realistically save?
For an image-heavy or scanned PDF, reductions of 60-90% are common while keeping the document perfectly readable at screen and normal print sizes. For a PDF that is already mostly text, there is little to gain — it is already small, and squeezing it further risks softening the few images it does contain for almost no benefit.
Set a target based on where the file is going. Under 25 MB clears Gmail and Outlook; under 10 MB is safe for stricter corporate mail; a 1-2 MB target suits most web upload forms. Aim to land comfortably under the limit rather than at the absolute minimum, so quality stays high.
The levers that actually shrink a PDF
Downsampling images is the big one: reducing embedded pictures to a resolution that still looks sharp on screen (around 150 DPI for screen use, 300 for print) removes data you will never see. Converting colour scans to grayscale cuts size sharply when colour is not needed. Removing embedded fonts you do not need and flattening form fields and annotations also helps at the margins.
What you should not do is repeatedly re-compress an already-compressed file — each pass degrades images without saving much. Compress once, from the original, to your target.