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A PDF can contain much more than visible text. It may include high-resolution photographs, scanned page images, embedded fonts, vector drawings, annotations, forms, thumbnails, metadata, transparency, attachments and objects that are no longer needed on the visible page. Because different PDFs are built differently, there is no single compression trick that guarantees the same percentage reduction for every document.

Adobe's current Acrobat guidance explains that optimization can reduce file size by working with images, fonts and unnecessary items. Its desktop PDF Optimizer also exposes separate controls for images, fonts, transparency, discarded objects, discarded user data and cleanup. For a professional reference, see Adobe's PDF Optimizer settings and Reduce PDF file size documentation.

Lossless vs lossy PDF compression

Lossless compression tries to reduce storage overhead without deliberately discarding visual information. It is safer for text, line art and documents where fidelity matters, but savings can be modest—especially if the source is already efficient. Lossy compression can discard image detail or encode page images more aggressively. That usually offers a better chance of shrinking scans and photo-heavy files, but visual quality can decline.

The modes on this page make that trade-off explicit. Light mode is the closest to a structural cleanup/rebuild approach. Balanced first tries a structure-preserving rebuild; when that is not worthwhile, Balanced, Strong and Extreme can render the visual pages and rebuild them as JPEG-based PDF pages. That can be effective for scans and image-dense PDFs, but it converts live PDF content into page images.

High-resolution scanned pages

Scanners can create page images at resolutions far above what is needed for normal on-screen reading. A 300–600 dpi color scan of many pages can become very large because each page behaves like a photograph. Downsampling and JPEG compression can make a big difference, particularly when the document is intended for screen viewing rather than high-quality reproduction.

Large photographs inside reports

Office documents, portfolios, brochures and reports often contain camera images that were inserted at their original resolution even though they appear much smaller on the page. The PDF may therefore retain far more image data than the visual page requires.

Embedded fonts and subsets

PDFs can embed fonts so documents render consistently on other systems. Font embedding is valuable, but many fonts, unusual character sets or inefficient embedding can increase size. Adobe's optimizer includes font-specific controls because this is a real source of file overhead.

Complex vector graphics and transparency

Maps, CAD-like drawings, illustrations, charts and transparency effects can create many PDF objects. These files may look simple at a glance while containing thousands of drawing commands. Rasterizing such pages can make the file smaller in some cases, but it also removes the advantages of vector content.

Forms, annotations, attachments and extra data

Interactive forms, comments, embedded files, multimedia, indexes, thumbnails and metadata can all add bytes. A professional optimizer can selectively remove some of these elements. Do not remove them blindly when they are part of the document's intended function.

The PDF may already be compressed

If a PDF has already been optimized, a second compression pass may deliver little improvement. Recompressing already compressed JPEG images can also lower quality without producing a meaningful size reduction. In that situation, the better choice may be to keep the original or target the source document before PDF creation.

When compression only removes redundant storage overhead, the visual difference may be negligible. When a tool downsamples or recompresses images, some detail can be permanently discarded. Adobe's PDF Optimizer documentation notes that JPEG and JPEG2000 quality settings can remove pixel data. The stronger the lossy image compression, the greater the risk of visible artifacts around text, diagrams, screenshots and photographs.

For this reason, there is no universal “best compression percentage.” A legal filing, a photographic portfolio, a scanned receipt and a lecture handout all have different requirements. Start with the least aggressive level that meets your size target. Open the compressed PDF and inspect every page before deleting the original.

What to check after PDF compression

• Text: Is small text still readable at normal zoom?
• Images: Are photos, signatures, stamps and screenshots clear enough?
• Pages: Did every page render in the correct orientation and dimensions?
• Searchability: If you used raster compression, remember that text is no longer stored as selectable live text by this tool.
• Links and forms: Rasterized modes flatten those interactive elements into the page image.
• Color: Check important brand colors, diagrams and shaded tables visually.

If a scanner stores every page as a large color image, even a simple black-and-white letter can consume far more storage than necessary. For on-screen reading, lowering render resolution and using efficient JPEG compression can reduce the number of bytes substantially. That is exactly the type of document for which Balanced or Strong mode can be effective.

However, OCR is a separate layer of functionality. A searchable scanned PDF may contain an invisible text layer over the scan. When this page uses raster compression, it rebuilds the visible page as an image and does not recreate the OCR text layer. If search, copy/paste, accessibility or text extraction matters, prefer Light mode or use a professional PDF optimizer that preserves OCR.

Color vs grayscale vs black-and-white scans

Color pages generally require more information than grayscale or bilevel pages. Professional scanning software can often reduce file size more intelligently by detecting text, choosing monochrome compression for black-and-white regions and retaining color only where needed. This browser tool intentionally keeps its workflow simple rather than pretending to replace full document-imaging software.

Image resolution and downsampling

Resolution determines how many pixels are used to represent the visual page or an embedded image. Downsampling reduces pixel count, which can dramatically reduce storage for scanned pages and photographs. The right resolution depends on viewing distance, screen size and printing needs. A document intended only for screen review can tolerate lower resolution than artwork intended for high-quality print.

JPEG quality and compression artifacts

JPEG is effective for photographs and scanned pages with continuous-tone imagery. At stronger compression, fine detail is removed and artifacts can appear around high-contrast edges. Text embedded inside a raster page can therefore look softer than live vector text. The Strong and Extreme modes on this page should be treated as delivery-copy tools, not archival masters.

Vector PDFs behave differently from scanned PDFs

A mostly vector PDF can already be compact because text and line drawings are described mathematically instead of storing a full image for each page. Rasterizing that PDF may actually increase file size while also sacrificing selectable text and sharp zooming. If you are compressing a born-digital PDF from Word, Excel, Illustrator, CAD or similar software, Light mode is the safer starting point.

Embedded fonts

Fonts can be embedded fully or as subsets containing only the characters used in the document. Subsetting often reduces size while preserving consistent rendering. Removing or unembedding fonts can create compatibility risks if the viewer does not have an appropriate substitute, which is why this browser tool does not offer a destructive “remove fonts” switch.

Forms, annotations and hyperlinks

Interactive elements are not just visual marks. A checkbox, text field, signature field or link is stored as a PDF object with behavior. Rasterizing the page captures how it looks but not the interaction. If your PDF contains forms, digital signatures, comments, bookmarks or important links, use Light mode and verify the result carefully—or use a professional optimizer designed to preserve those structures.

Accessibility and OCR

Accessible PDFs may include tagged reading order, alternative text, document structure and searchable text. Rasterizing pages discards that semantic structure in this implementation. Likewise, a searchable scan may lose its OCR text layer. If accessibility compliance or machine-readable text matters, use a workflow specifically designed to preserve tags and OCR.

Metadata and private information

Metadata can include title, author, subject, keywords and other document properties. It usually represents a small portion of total file size, so removing metadata alone rarely transforms a very large PDF. This tool offers a metadata setting mainly for document hygiene, not as the primary compression engine.

Print PDFs need different priorities

Commercial print files may rely on high-resolution images, CMYK/spot colors, bleeds, output intents, transparency and PDF/X requirements. Aggressive screen-oriented compression can make those PDFs unsuitable for production. For controlled print optimization, Adobe Acrobat Pro or the source design application is a better choice because you can tune individual image, font, transparency and compatibility settings.

Why exact compression percentages are impossible to promise

A 20 MB PDF containing uncompressed scans may shrink dramatically. A 20 MB engineering drawing full of complex vectors may not. A 500 KB text PDF may already be near its efficient size. Compression percentage is therefore an output of the document structure and settings—not a fixed property of a tool.

Best practice for everyday PDF compression

1. Keep the original PDF as a master.
2. Start with the least aggressive compression mode likely to meet your goal.
3. Compare the new size with the actual upload or email limit.
4. Inspect every page, especially small text, signatures, tables and diagrams.
5. If you rasterized the PDF, remember that searchability, live links and form controls are flattened.
6. Use a professional optimizer for archival, legal, accessibility, print-production or compliance-sensitive workflows.