If you've ever tried to email a PDF and hit a size limit, you've probably noticed something odd: two documents that look identical on screen can differ in size by 10x or more. That's because "PDF" isn't one format — it's a container that can hold wildly different things inside, and how it was created determines almost everything about its file size.
Why PDFs get bloated in the first place
Most oversized PDFs fall into one of three categories:
Scanned documents. A "scan to PDF" from a printer or phone app usually embeds each page as a full-resolution, uncompressed (or lightly compressed) image — often 300 DPI or higher, even if the page is just black text on white. A single scanned page can easily be 2-5MB; a 20-page scanned contract can hit 80MB.
Exports from design tools. PDFs exported from InDesign, PowerPoint, or Keynote often embed images at their original resolution and color depth, plus fonts, vector art, and sometimes unused embedded assets left over from editing.
Redundant embedded fonts. Every font used in the document — including every weight and style — can get fully embedded, sometimes duplicated across pages if the PDF was assembled by merging several source files.
What "compression" actually changes
A PDF compressor generally does some combination of:
Re-encoding images — recompressing embedded JPEGs/PNGs at a lower quality setting, or downsampling their resolution (e.g. from 300 DPI to 150 DPI, which is still sharp on a screen and fine for most printing).
Removing redundant data — stripping unused objects, duplicate font subsets, and metadata that serves no purpose once the document is final.
Restructuring the file — rewriting the PDF's internal object structure more efficiently, which by itself can shave off a meaningful chunk of size on files that were saved incrementally (edited and re-saved many times without a full rewrite).
Text itself is not "compressed" in the lossy sense — actual text characters are already tiny compared to images, so quality loss in a compressed PDF almost always shows up in images, not in the crispness of your words.
How to compress without visible quality loss
A few rules of thumb that avoid the "my PDF looks compressed" problem:
1. Start with structural compression first. Removing unused objects and redundant fonts costs zero visual quality — it's pure waste removal. If that alone gets you under your size limit, stop there.
2. Only downsample images if the document is screen-bound. If nobody is going to print the PDF at high resolution, dropping embedded images to 150 DPI is invisible on any modern screen but can cut file size dramatically.
3. Avoid re-compressing an already-compressed JPEG repeatedly. Every re-save of a JPEG at a lossy quality setting compounds artifacts. If your PDF is going to be compressed more than once (e.g., re-uploaded to different systems), it's worth compressing the source images once at a sane quality before ever putting them in a PDF.
4. For scanned documents, consider OCR + re-export instead of raw compression. A scanned page is fundamentally a photo of text. Running OCR (optical character recognition) and re-exporting as a proper text-based PDF can shrink a multi-megabyte scan down to a few hundred kilobytes, because you're storing actual characters instead of pixels.
Try it
GlaeKit's PDF Compressor runs entirely in your browser — it re-structures and optimizes the file locally, so nothing is ever uploaded to a server. Drop in a file and see the size difference before you download.
Frequently asked questions
Will compressing a PDF make the text blurry?
No — text in a PDF is stored as vector character data, not pixels, so it stays sharp at any zoom level regardless of compression. Blurriness after compression almost always comes from downsampled images, not text.
How much smaller can a PDF realistically get?
It depends entirely on the source. A well-optimized PDF exported directly from a text editor might only shrink 5-10%. A scanned document full of high-resolution images can often shrink 70-90% with no visible quality loss on screen.
Is it safe to compress a PDF containing sensitive documents?
It depends on the tool. Many online compressors upload your file to a server to process it, which means your document leaves your device. Browser-based tools that process everything locally — like GlaeKit's — never transmit the file anywhere.
Why does my PDF get bigger after I edit and re-save it?
Most PDF editors append changes rather than fully rewriting the file, to save time. Each edit adds new objects without removing the old ones, so a PDF that's been edited many times can accumulate a lot of dead weight — compressing it forces a full rewrite that discards the unused data.