Convert DJVU to PDF: Preserve the Scan, Test the Text Layer

DJVU-to-PDF is a page re-encoding, not a format swap

A DjVu-to-PDF conversion makes the document easier to open in browsers, mobile readers, and ordinary desktop software, but it does not simply change a label on the same bytes. DjVu is a scan-oriented format created at AT&T Labs in 1998. A compound DjVu page can store a high-resolution black-and-white mask for text and line art separately from lower-resolution foreground and background color layers. Its page data appears in an IFF-style chunk structure beginning with AT&T, with FORM:DJVU for one page or FORM:DJVM for a multipage document. PDF uses a completely different object model for pages, images, fonts, and text.

The converter therefore has to choose what it will make from each DjVu page. A simple route renders the composed page to one image and places that image on a PDF page. It often preserves the visible appearance well enough for reading, but it may discard the very features that made the source useful: its separate layers, compact compression strategy, bookmark information, and hidden OCR text. A better result is possible only when the tool deliberately transfers or rebuilds those parts. Do not judge the result by appearance alone. A PDF can look identical to its DjVu source while being much larger, unsearchable, or harder to copy from.

Layer fusion explains the surprising PDF size

DjVu can separate a scan into a text mask, foreground, and background before compressing them with methods suited to each part. The mask commonly uses JB2 data in an Sjbz chunk, while foreground and background use IW44 data in FG44 and BG44 chunks. When a conversion fuses those layers into one color image, the PDF no longer has the same split between sharp letters and soft paper texture. Wikimedia's DjVu conversion guidance documents the consequence: direct color conversions often become several times larger because a JPEG-style image compressor does not use DjVu's layered model efficiently.

A smaller PDF is not automatically a better one. Downsampling can make fine print unreadable, and aggressive JPEG compression can add blocks and halos around letters. The sensible check is to compare a difficult page at normal reading size and at high zoom: small serif type, maps, marginal notes, line drawings, and faint photographs show damage first. If the PDF is excessively large but still readable, a controlled image-size or quality adjustment is better than repeated conversion through several unrelated tools. Each new lossy raster pass has another chance to blur the scan; none can recreate detail that an earlier pass removed.

PDF opens more places but it inherits every scan limitation

• Gain - much wider viewing support: PDF opens directly in current browsers and mainstream operating-system viewers where DjVu normally needs a dedicated reader.
• Gain - easier sharing and printing: a PDF is the practical handoff for people who only need to read the scanned pages.
• Gain - fixed page appearance: a rasterized PDF shows the converted pixels consistently without relying on DjVu plug-in support.
• Loss - DjVu's layered efficiency: merging a mask and image layers into a regular PDF image can increase size by several times.
• Loss - possible hidden-text loss: a visual PDF may contain no selectable text if the conversion path ignores DjVu TXTz/TXTa data.
• Loss - no new recoverable detail: PDF cannot improve an original blurred scan, wrong OCR, or lossy IW44 background by changing containers.

PDF is the right destination when reader reach matters more than the compact source design. It is not the right promise if the request implies editable text, accurate OCR, or a smaller file. A DjVu scan with no text layer remains image-only after conversion unless OCR is added. A DjVu scan with poor OCR can become a searchable PDF whose copied text is still wrong. For research, accessibility, and text extraction, test the conversion with real words from several pages instead of trusting a check box labelled "searchable."

Viewer, printer and converter paths produce different PDFs

DjVuLibre is the open-source reference implementation. Its ddjvu decoder recognizes pdf as an output format, and it can also render selected pages, masks, foregrounds, backgrounds, or standard image formats. This is useful for controlled inspection, but the tool documentation describes decoded image output. It should not be assumed to carry every metadata or text feature across. A documented DjVuLibre feature request notes that ddjvu PDF output did not preserve the source text layer, and reports from PDF printer workflows show the same practical result: the visible scan arrives, but text is no longer selectable.

Printing from a DjVu viewer is another route, not a guarantee. A virtual PDF printer receives what the viewer sends to the print system; it may receive only the rendered page image and never see the invisible OCR layer. A viewer's own "Export as PDF" command may do better or worse depending on its implementation. Test each route after conversion: search for a distinctive word, select it, copy it into a plain-text editor, inspect the PDF bookmarks, and compare the total page count. These five checks reveal more than a successful progress bar. If preserved text and bookmarks are essential, use a path explicitly designed to transfer them or add OCR to the PDF afterward and label it as new OCR, not original text.

The source's page type also changes the right test. A pure black-and-white technical scan may survive a direct render with a modest file increase, while a color magazine page exposes the cost of collapsing DjVu's background, foreground, and mask. A document with a NAVM outline needs a bookmark check, not just a page-count check. A bundle that uses shared data needs every page opened before conversion is trusted. This is why two files with the same .djvu extension can produce very different PDFs: the extension names the container family, not the presence of OCR, bookmarks, color layers, or intact page components.

Four output faults with evidence-based fixes

The PDF looks right but searching returns nothing. It is image-only or the DjVu hidden text was not transferred. Confirm by copying a visible word into a text editor; if nothing meaningful arrives, run OCR on the PDF and proofread important pages. The new PDF is far larger than the source. Layer fusion is the likely reason: DjVu's separate JB2 mask and IW44 image layers became a combined raster image. Re-render at a tested resolution or quality setting once, then compare fine text before replacing the original. Do not repeatedly reconvert the already-converted PDF.

Copying text produces wrong characters or a scrambled order. The source OCR can be inaccurate, especially in old type, multi-column layouts, or mixed languages. The conversion may have transferred that same bad layer perfectly. Extract it for editing only after comparing it against the scan, or rerun OCR with the correct languages and a cleaned page image. Some pages are blank, rotated, or missing. Verify the original in a second DjVu viewer. A page-specific corruption or a reader's incomplete rendering support can produce a conversion fault that is not in every page. For recovery-oriented decoding, DjVuLibre provides a -skip option that can continue past corrupted pages; keep the original and record exactly which pages need replacement.

One scanned page, two containers, six practical differences

Questions before publishing a DjVu-derived PDF

Will the PDF always be smaller or the same size?
No. Color DjVu pages often grow after their layers are flattened into standard PDF page images. The size depends on resolution, image quality, and the conversion path.

Does selectable PDF text prove that the book was originally digital?
No. It may be an OCR layer transferred from DjVu or newly created after conversion. Check copied passages against the visible scan before treating it as authoritative text.

Can printing a DjVu file to PDF preserve the hidden text?
Not reliably. A print route can receive only rendered image data. Test search and copy in the actual output, or choose a tool documented to handle text-layer transfer.

Should the DjVu be retained after a successful PDF conversion?
Yes. It remains the compact source with its original page-layer structure and may retain text or outline data that a PDF export omitted.