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Why is my large TIFF to PDF conversion slow? (and how to fix it)

By DeckleUpdated

If a large TIFF conversion in TIFF to PDF seems stuck or stops, check the TIFF's pixel size and number of frames, then convert it on a computer, with fewer frames per run, or from a lower-resolution copy. Choose Cancel on the progress screen to stop a run at any time. Nothing is lost: your TIFF is not changed, and you can start again straight away.

TIFF to PDF

One file can hold several frames

Frame 1

Cover

Frame 2

Table

Frame 3

Signed page

3 included frames → 3 planned PDF pages

Example · count included decoded frames, not TIFF filenames.

How to get a large TIFF through

  1. Find the TIFF's pixel size, such as 4960 × 7016, in your image viewer's file information, or work it out from the scan resolution in the table below.
  2. Multiply the two pixel numbers. Over 16 million (16 megapixels) on a computer or Android device, or 4 million on an iPhone or iPad, each frame is scaled down to that size, which takes longer. Over 64 million, or 25 million on an iPhone or iPad, the file is refused before you start.
  3. If the frames are much larger than you need, reduce the resolution in an image editor or rescan at 300 DPI.
  4. If there are many frames, split the work: convert a few TIFF files per run, or save the TIFF as several smaller files.
  5. Open TIFF to PDF and add the TIFF, or drop it onto the page.
  6. Close other heavy tabs and apps, then choose Create PDF.
  7. Join the PDFs from several runs with Merge PDFs.

Why large TIFFs are slow

Deckle converts in your browser, on your device. For every frame it decodes the full image into memory, paints it onto a white background, saves it as a quality-90 JPEG and writes it into the PDF. It does this one frame at a time, so memory use depends mostly on the size of each frame, while total time depends on the number of frames.

Rough sizes for a single A4 page:

Scan resolution Pixels per frame Megapixels On a computer
200 DPI 1654 × 2339 3.9 Quick
300 DPI 2480 × 3508 8.7 Fine
400 DPI 3307 × 4677 15.5 Close to the limit
600 DPI 4960 × 7016 34.8 Scaled to 16 megapixels

A 200-frame TIFF of 300 DPI pages is well within limits per frame, but it is 200 decodes and encodes in a row, so it takes a while. The progress screen counts files, not frames, so a single long TIFF can show "0 of 1 image" for most of the run. That is normal; leave it running, or choose Cancel to stop.

When the run stops with an error

Deckle stops rather than producing a PDF with pages missing. The message names the file:

  • Too large to read. A frame above 64 megapixels, or 25 megapixels on an iPhone or iPad, is caught when you add the file: Create PDF stays disabled and names it. Reduce the resolution of the TIFF, or convert on a computer if you are on an iPhone or iPad. Frames between the per-page limit and that ceiling are scaled down, not refused, and the review lists them.
  • Unsupported compression. Uncompressed, LZW and Deflate TIFFs are read; fax (CCITT), PackBits and JPEG compression are refused. Re-save with LZW.
  • CMYK colour. CMYK TIFFs are refused. Re-save as RGB or greyscale.
  • Not enough memory. On a device with little free memory, the browser can refuse a large image. Close other tabs and try again, or use a computer with more memory.

Make the TIFF lighter

For scanned documents, 300 DPI is plenty for reading and printing. An image editor can reduce a TIFF's resolution; check that the saved copy still has every frame. If the file came from a scanner, rescanning at 300 DPI greyscale is often quicker than editing.

Splitting also helps. A 400-frame archive is easier as four runs of 100 frames: each run is shorter, a failure costs less, and you can merge the four PDFs at the end in Merge PDFs.

Check the PDF size before you send it

Frames are saved as JPEG, so a colour scan usually gives a PDF of a similar size to the TIFF or smaller. A crisp black-and-white TIFF can grow, because JPEG stores the page as greyscale tones rather than pure black and white. If a 50 MB TIFF becomes a much larger PDF, scanning in greyscale at a lower resolution helps more than anything else.

Tips

  • Convert on a computer. iPhone and iPad keep 4 megapixels per frame, so full-page scans above 200 DPI are scaled down there, and frames above 25 megapixels are refused.
  • Leave the tab open. Closing or reloading the tab stops the run.
  • Start with a small run. Convert one TIFF first to see how long a frame takes, then plan the rest.
  • Use Cancel freely. Cancelling discards only the unfinished PDF; the original TIFF stays as it was.

Frequently asked questions

Is there a limit on the number of frames?

There is no fixed frame limit. Each frame must be under 64 megapixels (25 on iPhone and iPad), and a very long run is bounded by your device's memory and patience.

Why does my phone refuse a TIFF my computer converts?

iPhone and iPad refuse frames above 25 megapixels to keep the browser from running out of memory, and scale anything above 4 megapixels down. Computers and Android devices accept frames up to 64 megapixels and keep up to 16 megapixels per page.

Will closing the tab lose my TIFF?

No. The TIFF on your device is never changed. Only the unfinished PDF is lost.

Can I make the conversion faster?

Use fewer, smaller frames: scan at 300 DPI or lower, in greyscale for text, and split long archives across several runs.

Related tools

  • TIFF to PDF to convert scans one frame per page
  • Merge PDFs to join PDFs from several runs
  • Split PDF to break a long scanned PDF into parts afterwards

Sources