By default, Fillable PDFs sends a URL to our API when an image file is to be embedded into a PDF. Our API then attempts to download the image from that URL. This filter allows you to instead enable the sending of a Base64 encoded image binary to the Fillable PDFs API.
This is especially helpful to use if your site is a local installation, behind a basic authentication login, using an aggressive firewall configuration, or you are otherwise having issues embedding images using the default URL method.
Note: You will be unable to use this filter to embed GIF files. GIF image binaries are too large to transmit to our API, in the majority of cases, within the execution time we allow for. A timeout is very likely to occur.
add_filter( 'fg_fillablepdfs_use_image_binary_pre_generate', '__return_true' );
This filter was added in Fillable PDFs 3.1.4