Blurry text after PDF compression almost always comes from one of two mistakes: applying compression that affects the text layer instead of just the images, or pushing image compression too aggressively without understanding what the document actually contains. Both are fixable once you understand how PDF compression works.
What PDF compression actually does
A PDF file contains multiple types of content and compression affects each type differently. Text in a properly created digital PDF is stored as vector data: mathematical descriptions of letter shapes that can be rendered at any size without quality loss. Compressing a PDF should never affect this text layer. Text compression in PDF refers to compressing the encoding of the text data, which has no visual effect.
Images embedded in a PDF are stored as raster data: grids of pixels. These are where meaningful compression with visible quality tradeoffs occurs. A PDF containing scanned pages is essentially a collection of large images in a PDF container. Compressing these images reduces file size but can reduce image quality if pushed too far.
When PDF compression makes text blurry, it usually means one of two things happened. Either the original PDF was a scanned document where the entire page is an image (so the text was always part of an image, not a text layer), or a compression tool was applied that flattened the entire document to images and then compressed those images too aggressively.
Diagnosing your PDF before compressing
Try clicking on text in your PDF reader. If you can select text by clicking and dragging, the text is a real text layer. This is a digital PDF. Compression will affect embedded images but should not visually affect the text.
If clicking on the text does not select it, or if the entire page is treated as a single element, the PDF is scanned. The text is part of an image. Compression will visually affect it, and the degree of quality loss is determined by how aggressively the image compression is applied.
Compressing a digital PDF with text layers
For a digital PDF from Word, Google Docs, LibreOffice, or any design application, the main targets for compression are embedded images within the document. A digital CV that contains a professional headshot, a report with charts and graphs, or a brochure with product photography will have large embedded images driving the file size.
Compressing a digital PDF should never make text blurry because the text is not being compressed as an image. A good compression tool targets the embedded images while leaving the text vectors untouched.
The most common reason digital PDFs are larger than expected is the export quality setting used when the PDF was created. A Word document exported with "Best quality for printing" produces a PDF optimised for 300 DPI printing. The same document exported with "Minimum size" or "Screen quality" produces a PDF designed for on-screen reading at approximately 72 to 96 DPI, which is significantly smaller without any visible difference when read on screen.
If you cannot re-export from the source application, running the PDF through the PDF tools compression with a moderate setting will compress embedded images while leaving text intact.
Compressing a scanned PDF without losing readability
Scanned PDFs require more careful handling because every page is an image. Compressing too aggressively creates blurry text because you are reducing the quality of the image that contains the text.
The key variable for scanned PDFs is the effective resolution of the page images. A scan at 300 DPI stores significantly more information than a scan at 150 DPI, but for a document that will only be read on screen, 150 DPI provides adequate text readability. Reducing a 300 DPI scan to 150 DPI equivalent while compressing reduces file size by approximately 75 percent.
Colour mode is the other major variable. A scan of a black and white document saved in full colour stores three colour channels (red, green, blue) for every pixel, even though the useful information is only black or white. Converting to grayscale reduces the data to one channel per pixel, cutting the image data to one-third and producing a file that is typically three to four times smaller. The visual appearance of a grayscale scan of a black and white document is identical to the colour version.
The right approach for a scanned PDF is to use grayscale conversion alongside moderate compression rather than applying aggressive JPEG compression to a full-colour scan. The grayscale conversion alone achieves the majority of the size reduction, leaving less work for the compression algorithm to do and less quality to be sacrificed.
When compression causes blurring and how to fix it
If you have already compressed a PDF and it is blurry, you cannot reverse the compression on the file you have. Compression permanently discards image data. The fix requires going back to the original file.
If you have the original scanned pages, try compressing with a lighter setting or using the grayscale approach described above rather than heavy JPEG compression. The goal is to get most of the size reduction from colour mode change and moderate compression rather than from aggressive quality reduction.
If you do not have the original uncompressed source, the blurry compressed PDF is what you have to work with. In this case, the only option is to explain the situation to the recipient and try to obtain a fresh version if one is needed.
Settings that keep text sharp in different scenarios
For a typed certificate or formal letter in digital PDF format: use light compression targeting embedded images only. The text will remain perfectly sharp. Expected size reduction from a 1MB file is approximately 30 to 50 percent if the document contains embedded images.
For a scanned certificate in black and white at 300 DPI colour: convert to grayscale, apply moderate JPEG compression to the resulting grayscale images. Expected size reduction from a 5MB file is 70 to 85 percent with text remaining clearly readable.
For a mixed document with both typed sections and scanned pages: handle the two types separately if possible, then combine. Use Split PDF to separate scanned pages from digital pages, compress each appropriately, then Merge PDF to reassemble.
When not to compress at all
Some documents should not be compressed or should only be compressed minimally. Legal documents where every character must be unambiguous, medical imaging reports where pixel values may be clinically significant, and technical engineering drawings where measurement marks must be precisely readable are all examples where aggressive compression creates unacceptable risk.
For these documents, splitting is almost always preferable to compression. Splitting a 15-page technical document into three parts of five pages each meets a 5MB portal limit without touching the quality of the content.