pdfjoin:tldr:61c6e

This command is using the "pdfjoin" utility to merge multiple PDF files into one file. Let's break down the command step by step:

1. "pdfjoin": This is the name of the utility or command-line tool that is being used to join PDF files.

2. "${filename1-pdf filename2-pdf ---}": This part is likely representing a list of input PDF files that you want to merge. The specific file names depend on the context in which this command is being used. The ellipsis (...) suggests that there could be additional filenames included.

3. "${1}": The number "1" within curly braces represents a command-line parameter or argument. Again, the specific purpose of this parameter depends on the context. It is often used to denote the first command-line argument provided when executing the script.

4. "--outfile": This flag is used to specify the output file name or path for the merged PDF file that will be generated.

5. "${output_file}": Similar to the first curly braces, this likely represents a variable or command-line argument that specifies the desired name for the output file.

Overall, this command takes multiple PDF files as input, combines them into a single document, and creates a new PDF file as the output. The specific filenames and arguments should be filled in accordingly based on the requirements of the task or script that uses this command.