Why Is a PDF File Smaller Than Its Extracted Images?
Question details
The user is confused about how a small PDF file can produce extracted images that total a much larger file size, and why recreating the PDF makes a larger document.
- Product
- Device & OS
- not provided
- Scenario
- Extracting images from a PDF or recreating a new PDF using previously extracted images.
- Observed behavior
- The original PDF is only 268 KB, but the extracted images total 7.51 MB. Attempting to recreate the PDF with those images results in a significantly larger file than the original.
Before analyzing the file size discrepancy, ensure you have an advanced PDF editor installed to audit the document's space usage and compression properties.
Analyze PDF Compression and Extraction Settings
Identify how PDF compression algorithms reduce file size and how extraction methods can inadvertently inflate image formats.
PDFs use advanced internal compression algorithms (such as JPEG, ZIP, or JPEG2000) and resolution downsampling to significantly reduce file size without noticeably degrading visual quality. The discrepancy occurs because extraction tools often decode these highly compressed embedded images and save them in uncompressed or less efficient formats (like BMP, high-quality PNG, or uncompressed TIFF).
Additionally, when recreating the PDF, if the original strict compression settings, color space reductions, and downsampling options are not manually applied, the new PDF will retain the larger size of the extracted images.
Open the original PDF in an advanced editor like Adobe Acrobat. Navigate to the PDF Optimizer or Document Properties and select 'Audit Space Usage' to verify exactly how many bytes the embedded images are consuming.
Review the settings of the tool you used to extract the images. Ensure it is extracting the native raw stream (e.g., extracting a JPEG as a JPEG) rather than converting the image into a lossless format.
Check if the original PDF stored the images in grayscale or indexed color. If your extraction tool converts them to 24-bit RGB, the file size will drastically increase.
When recreating the PDF, open your PDF creation settings. Ensure image downsampling is enabled (e.g., downsample to 150 PPI) and set the image compression format to JPEG with medium or low quality.
Optimize and Extract PDF Images Using WPS Office
WPS Office provides built-in PDF tools to view, compress, and extract images while giving you control over file size and document quality. Manage your PDFs effortlessly with its lightweight and user-friendly interface.
- 1. Open your PDF: Download and install WPS Office Free, then open your PDF document using WPS PDF.
- 2. Compress PDF: To recreate a smaller PDF, navigate to the 'Tools' tab and select 'Compress PDF' to adjust image quality and downsampling.
- 3. Extract Images: To pull images out properly, use the 'Extract Images' feature or 'PDF to Picture' under the Tools menu, selecting an efficient output format like JPEG.
Frequently Asked Questions
Why do images extracted from a PDF get so large?
Images inside a PDF are often heavily compressed or downsampled. When extracted using third-party tools, they are frequently decoded and saved as uncompressed formats like high-quality PNGs or TIFFs, which drastically increases their file size.
How can I recreate a small PDF from large extracted images?
To recreate a small PDF, you must enable image downsampling and compression during the PDF creation process. In your PDF software's save or export settings, choose to compress images using JPEG format and reduce the resolution to 150 DPI or lower.
Does changing the color profile affect PDF image size?
Yes. A PDF might store an image in grayscale or indexed color to save space. If an extraction tool automatically converts these embedded images into 24-bit RGB or 32-bit CMYK upon saving, the resulting image file will be significantly larger.
