There are six panels where you select advanced options for PDF optimization: Compress Images, Unembed Fonts, Transparency, Discard Objects, Discard User Data, and Clean Up.
The Compress Images panel
In the Compress Images panel, you can select options to specify downsampling and compression for color, grayscale, and monochrome images.
Downsample: Select a downsampling method, and set the appropriate image resolution values. For images with higher resolution than the For Images Above value you input, reduce the image resolutions to the specified resolution by merging pixels in a sample area of the image to make one larger pixel.
- Off – Turn off downsampling.
- Average Downsampling - Average the pixels in a sample area and then substitute the entire area with the average pixel color at the resolution you specified.
- Subsampling - Select a pixel from a sample area and then substitute an entire area with the selected pixel at the specified resolution. This method reduces conversion time as compared with downsampling, but the generated images are less smooth and continuous.
- Bicubic Downsampling - Use a weighted average, instead of a simple average (as in Average Downsampling) to decide pixel color. This method increases conversion time but produces the smoothest tonal gradations.
Compression: Reduces file size by removing unnecessary pixel data. You can specify a compression standard to process images. Generally, apply ZIP, JPEG, or JPEG 2000 compression to color/grayscale images, and ZIP, JBIG2, CCITT Group 3 or 4, or Run Length compression to Monochrome images. You can make your choices by following these rules:
- JPEG and JPEG 2000 compression give better results on images like photographs with gradual transitions from color to color. JPEG 2000 produces a higher quality final image, even when using lossy compression.
- Because JPEG compression eliminates data, it can achieve much smaller file sizes than ZIP compression.
- ZIP is the better choice for illustrations with large areas of single colors or repeating patterns, and for monochrome images that contain repeating patterns.
- CCITT (Consultative Committee on International Telegraphy and Telephony) compression is available only for monochrome bitmap images. CCITT Group 4 is a general-purpose method that produces good compression for most monochrome images. CCITT Group 3, used by most fax machines, compresses monochrome images one row at a time.
- JBIG2 (Lossless)/ JBIG2 (Lossy) is appropriate for bi-level or monochrome images which is suitable for lossless or lossy compression. For monochrome images, JBIG2 compression is superior to CCITT.
- Run Length produces the best results for images that contain large areas of solid white or black.
- High Compression applies to almost all images. If you intend for small-size images and don’t care about the quality, High Compression can produce smaller images than other compression methods do. (Available in Foxit PDF Editor Pro Only)
Tile Size: Divides the image being compressed into tiles of the specified size. (If the image height or width is not an even multiple of the tile size, partial tiles are used on the edges.) Image data for each tile is individually compressed and can be individually decompressed. The default value of 256 is recommended. This option is available only for JPEG 2000 format.
Quality: Available only for JPEG compression, JPEG 2000 compression, High Compression, and JBIG2 compression. You can apply JPEG compression, JPEG 2000 compression, or High Compression to color and grayscale images at various levels (Minimum, Low, Medium, High, Maximum). JPEG and JPEG 2000 compression methods are typically lossy, which permanently remove some pixel data and may reduce image quality. JPEG 2000 compression also offers lossless compression which means no pixel data is removed. Compression for monochrome images is lossless, except for JBIG2 compression and High Compression.
Compress images only if there is a reduction in size: With the option selected, images will not be optimized if there are no reductions in file size from the image setting.
The Unembed Fonts panel
Embedding all fonts allows anyone who opens your file to see the document as you intended it to be seen. Without embedding fonts, the PDF viewer will substitute a font with a close match or a default font which is available on the computer, and the display results may be not exactly what you intended. If you don’t mind the difference and prefer a smaller file, you can unembed fonts.
This panel contains two lists for fonts: fonts that are available for unembedding, and fonts to unembed. To unembed fonts in a document, do as the following:
- Select the checkbox for the panel first.
- Select one or more fonts you want to unembed in the Embedded fonts list, and click the Unembed>> button to add the fonts to the Fonts to unembed list. If you want to only embed the characters (a subset of a font) used in the document, select the Subset all embedded fonts option.
- (Optional) To remove the added fonts from the Fonts to unembed list, select the font and click the <<Retain button. If you don't want to unembed any font in the document, select Do not unembed any font.
The Transparency panel
If your document includes content that contains transparency, you can use the preset settings in the Transparency panel to flatten transparency to help reduce the file size.
The Discard Objects panel
The Discard Objects panel lists options that allow you to remove objects from the PDF.
Discard all form submission, import and reset actions: Disables all actions related to submitting/importing form data and resetting form fields. This option retains form objects to which actions are linked.
Flatten form fields: After flattening, all form fields can no longer be filled or modified. Only the items that are visible with the Hand command selected remain and change into shape objects.
Discard all JavaScript actions: Removes all actions in the PDF that use JavaScript. This option is selected by default.
Discard all alternate images: For an image that includes multiple versions, removes all versions except the one used for on-screen viewing. Different versions may be used for different purposes, such as low-resolution on-screen viewing and high-resolution printing.
Discard embedded page thumbnails: Removes page thumbnails embedded in the document. See also Embed page thumbnails.
Discard document tags: Removes all tags from the document, which will have an impact on the document accessibility. See also the Accessibility chapter.
Convert smooth lines to curves: Turns lines into curves, which can help reduce the file size and make on-screen rendering faster.
Detect and merge image fragment: Detects image fragments in the document, and merge the adjacent image fragments with the same image properties into an image object if the number of fragments is more than 25.
Discard embedded print settings: Removes print settings embedded in the PDF, such as page scaling and duplex mode.
Discard embedded search index: Removes the search indexes embedded in the document.
Discard bookmarks: Removes all bookmarks from the document.
The Discard User Data panel
This panel allows you to specify whether to remove personal information including comments, attachments, layers, and document metadata.
Discard all comments, forms and multimedia: Removes all comments, forms, form fields, and multimedia from the document.
Discard external cross references: Removes links to other documents, not including the links that go to pages within the PDF.
Discard document information and metadata: Removes data in the document information dictionary and all metadata streams, such as the information in the File > Properties > Description.
Discard all object data: Removes all the user attribute information set in tag nodes.
Discard file attachments: Removes all file attachments, including the attachments that are added to the PDF as comments.
Discard private data of other applications: Removes the information in PDF documents that is related only to the application that created the PDF documents.
Discard hidden layer content and flatten visible layers: Remove all layers’ information. All layers in the Layers panel will be removed.
The Clean Up panel
Use the Clean Up panel to remove useless items (obsolete or unnecessary elements) from the document. Removing certain elements can seriously affect the functionality of the PDF. By default, only elements that do not affect document functionality are selected.
Object compression options: Specifies how to apply compression in the document.
Use Flate to encode streams that are not encoded: Applies the Flate compression to all streams that aren’t encoded in the document.
In streams that use LZW encoding, use Flate instead: Applies the Flate compression to all content streams and images that use LZW encoding in the document.
Remove invalid bookmarks: Removes invalid bookmarks that go to the pages that have been deleted in the document.
Remove invalid links: Removes links that jump to the destinations that no longer exist.
Discard unreferenced named destinations: Discards named destinations that are not being referenced internally from within the document.
Optimize page content: Removes all newline characters (or end-of-line characters), which can improve the Flate compression.
Optimize the PDF for fast web view: Restructures the document for page-at-a-time downloading from web servers, which allows users to open and view the PDF even before the whole file is downloaded.