How We Optimize PDFs
PDF compression on PDFCheck rewrites your file with Ghostscript, an open-source PDF and PostScript interpreter, using one of four presets. Savings depend on the file; image quality can change. This page lists the exact settings, what they do to a document and what you should check before using the result.
What compression changes
- Image resolution and image compression, according to the preset you choose
- Embedded fonts, which are compressed and reduced to the characters used
- Repeated images, which are stored once
- The whole file structure, which is rewritten as PDF 1.4
What it does not do
- It does not linearize files for fast web viewing or create object streams
- It does not run OCR, remove pages or change the text you see
- It does not guarantee a smaller file: some outputs are larger than the original
Step 1: Upload and checks
The upload passes the same gate as the other tools: the file must be detected as a PDF, start with a valid header and not look like an archive or an executable. The original is then saved to private storage under a random token so Ghostscript can read it. Files that need a password to open cannot be processed; remove the password first with the PDF Password tool if you are allowed to.
Step 2: The Ghostscript command
Every job uses the pdfwrite device with these settings: compatibility level 1.4, duplicate image detection, font compression and font subsetting. The preset you pick sets Ghostscript's PDFSETTINGS value, which controls image downsampling and image compression:
- Screen (72dpi) uses the screen preset. Images are reduced to about 72 dpi, which usually gives the smallest file but visibly softer photos and scans.
- eBook (150dpi) uses the ebook preset, with images at about 150 dpi. It suits reading on screens and most email attachments.
- Printer (300dpi) uses the printer preset, with 300 dpi image settings. It keeps more detail and saves less space.
- Prepress (300dpi) uses the prepress preset for print production. It is the most conservative choice, but the document is still rewritten and image data can still change.
Optional settings
Convert to grayscale tells Ghostscript to convert all colour to gray. Colour images, charts and highlighted text become shades of gray, which can save space but cannot be undone in the output.
Clear document information fields blanks Title, Author, Subject, Keywords and Creator after the input is read. Ghostscript writes its own Producer value into the new file. This option does not remove text, annotations, attachments or other content; use the Metadata Remover for a fuller review of what a file reveals.
Step 3: Measuring the result
Each job may run for up to five minutes. When it finishes, PDFCheck compares the original and output sizes and shows the difference and the percentage saved. The output is offered even when it is larger; the original is never replaced. Image-heavy PDFs may benefit from lower image resolution. Text-only or already compressed PDFs may shrink little or become larger.
What can be lost
Because the whole file is rewritten, any digital signature is invalidated, and a PDF/A or PDF/UA declaration should not be assumed to survive; validate the output again if you need either. Lower presets can make small text in scanned pages, fine lines in drawings and photo detail harder to read. Check links, bookmarks and fillable form fields in the output before sending it, and keep the original. Compare the output size and appearance before using it.
What to expect
Typical outcomes by document type. Actual savings depend on how the original was made.
| Document | Likely result | Suggested preset |
|---|---|---|
| A scan or photo-heavy brochure exported at high resolution | Often a large reduction, because images are downsampled | eBook (150dpi), or Screen (72dpi) for viewing only |
| A text report from a word processor with few images | Small change; font subsetting may save a little | eBook (150dpi) |
| A file that has already been compressed | Little or no saving, sometimes a larger file | Keep the original if the output is larger |
| A print-ready file for a printer | Moderate change; colour profiles and fine detail matter more than size | Prepress (300dpi), then check the proof |
Limits and file handling
The Free plan accepts files up to 50 MB, and each compression uses one of its 100 daily checks; Pro and AI plans accept files up to 100 MB. The original and the optimized copy are stored in private storage. The download link expires 24 hours after upload, and an hourly cleanup removes working files older than 24 hours.
Compress a PDF
Compress PDFs with adjustable presets. Savings depend on the file; image quality can change.
Optimize PDF