Lob Help Center

Mail piece file & font formats

File formats


We highly recommend designing your direct mail in HTML, as opposed to static formats. The flexibility afforded by HTML includes the incorporation of dynamic merge variables into the design, which enable sending personalized content at scale. See dynamic personalization for more information.

When designing content with HTML, use inline styles or an internal stylesheet. Do not use an external stylesheet. If you are linking to images, please ensure that they are at least 300 dpi (but there is no need to go higher than 300 dpi). Because the content is only being designed for a single size, we recommend using absolute positioning, as shown below.

body {
  width: 6.25in;
  height: 4.25in;
  margin: 0;
  padding: 0;
  background-image: url(https://s3-us-west-2.amazonaws.com/public.lob.com/assets/beach.jpg);
  background-size: 6.25in 4.25in;
  background-repeat: no-repeat;

#safe-area {
  position: absolute;
  width: 5.875in;
  height: 3.875in;
  left: 0.1875in;
  top: 0.1875in;
  background-color: rgba(255,255,255,0.5);

PDF guidelines

Lob processes and prints various types of PDFs, however the below PDF formatting guidelines will help ensure successful and accurate printing. 

  • All generated PDFs need to be PDF/X-1a compliant
  • Transparencies should be flattened. If transparencies are not flattened, overlapping colors may change and objects may disappear completely
  • Fonts must be outlined (preferred) or embedded. Fonts that have not been outlined or embedded might change or render incorrectly, specific letters may be substituted with incorrect glyphs
  • File size should be under 5MB. Lower file size allows for easier processing at the print level. Large files have a higher potential to fail
  • Color should be CMYK Color Space (SWOP v2 or GRACoL 2006 preferred). By submitting CMYK you will avoid potential conversion errors when Lob changes RGB to CMYK
  • Image resolution should be 300 ppi. If resolution is below 300 ppi, you may see pixelation and incorrect color saturation of your images, if ppi greatly exceeds 300 the file size will be too large and have the potential to fail
  • NO printers marks. Do not submit printer marks (or trim lines) including, but not limited to crop marks, trim marks, bleed marks, slug lines, registration marks, color bars, page information, etc. 

Avoid printer marks

Lob's APIs will not accept printer marks (or trim lines). Our production facilities have several QA standards in place to ensure cuts are made appropriately and the color is correct. Any content uploaded is treated as part of the design, and thus the printer marks will cause the dimensions to be larger than the intended image size, resulting in an error.

If you are exporting an image for a printing proof the way you usually would when sending images to traditional printers, then the trim lines should be unticked in your settings to ensure a sizing error does not occur. 

To remove these trim lines and color bars from your PDF, open your file in Adobe Acrobat, go to Settings, and clear the Printer’s Marks checkbox.

Avoid having any printer's marks in your PDF by ensuring your image export settings do not include them. 

Note that whether or not you export with “Bleed” will depend on how you built your template. If you already manually added the bleed (e.g. 4.25" x 6.25" artwork), then you should have that unticked as well.

Font formats

Supported fonts

Ideally, all fonts in provided PDFs should be embedded. Embedding a font in a PDF ensures that the final printed product will look as it was designed. Fonts can vary greatly in size and shape, even within the same family. If the exact font that was used to design the artwork is not used to print, the look and placement of the text is not guaranteed to be the same.

In general, requests that provide PDFs with un-embedded fonts will be rejected. We make an exception for "standard fonts", a set of fonts that we have identified as being common. PDFs that contain un-embedded fonts that are found in the list, and match the accepted font type will be accepted. Otherwise, the request will be rejected.

We support the following web font formats:

  • TTF
  • OTF
  • SVG
  • EOT
  • Google web fonts
  • Custom fonts within your HTML templates
  • See our full list of supported PDF Fonts

Unsupported fonts

We do not support Adobe (Typekit) fonts

  • Adobe Fonts (formerly Typekit Fonts): These fonts require white-listing a specific domain, which Lob is unable to do. Our recommendation is to either host the font yourself and use it within your HTML, or find an appropriate non-Typekit font to replace it with. 
  • Make sure all non-standard fonts must be embedded.

Image formats

When using PNGs or JPEGs with Lob, we require a minimum of 300 dpi. The dpi is calculated as (width of image in pixels) / (width of product in inches) and (length of image in pixels) / (length of product in inches). For example: 1275px x 1875px image used to create a 4.25" x 6.25" postcard has a dpi of 300. It is also recommended that you don't greatly exceed 300 dpi, as this will result in unnecessary additional file size.

Submitted images must have the same length to width ratio as the chosen product. Images will not be cropped or stretched by the API.


This is a raster image format that can have a transparent background. It is generally a higher quality than other image formats.


This is a raster image format that is often used for photographs. It does not allow for a transparent background.

Hosting content

When you pass an image or send HTML in your API request, Lob will then render and host the content. If you are sending at high volumes, we recommend you host the content yourself on a performant file hosting provider, such as Amazon S3, and send Lob a hosting URL to the content in your API request. This will reduce your API request time.

Rendering best practices

To see how your HTML is rendered, create a Test API request either through the API or on the Dashboard. You'll be able to see a PDF proof of exactly what the design looks like. We highly recommend sending yourself a printed piece to validate the mail piece appearance.

Example pieces

See Lob's Template Gallery for pre-designed templates to help you get started. You can fork these to create your own designs.


Lob's HTML renderer is based off of Webkit. For this reason, using the Safari browser will show previews more accurately. For the best results, we recommend submitting test API requests and reviewing the rendered PDF.


We highly recommend using the Adobe Creative Suite (Adobe Illustrator, Photoshop, or InDesign) to design your content. This will give you design flexibility and multiple export options.

Converting from PDF to HTML

We recommend using a developer to convert your PDF designs to HTML. We'd estimate 15 minutes to 2 hours of development time for the average file. Some applications like Adobe Illustrator come with HTML export options, but note that their exports won't typically produce HTML that conforms to Lob's requirements.

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