Question

I want to pass all the files as a single argument on Linux but I am not able to do that.

This is working

ls | sort -n | xargs  -i pdftk  {} cat output combinewd2.pdf

This passes a single argument per command, but I want all in one command.

Answer 1

This is one way to do it

pdftk $(ls | sort -n) cat output combinewd2.pdf

or using backtick

pdftk `ls | sort -n` cat output combinewd2.pdf

For example, if the filenames are 100, 2, 9, 3.14, 10, 1 the command will be

pdftk 1 2 3.14 9 10 100 cat output combinewd2.pdf

To handle filenames with spaces or other special characters consider this fixed version of @joeytwiddle’s excellent answer (which does not sort numerically, see discussion below):

#-- The following will handle special characters, and
#   will sort filenames numerically
#   e.g. filenames 100, 2, 9, 3.14, 10, 1 results in 
#      ./1 ./2 ./3.14 ./9 ./10 ./100
#
find . -maxdepth 1 -type f -print0 |
  sort -k1.3n -z -t '\0' |
  xargs -0 sh -c 'pdftk "$@" cat output combinewd2.pdf' "$0"

Alternatives to xargs (bash specific)

xargs is an external command, in the previous example it invokes sh which in turn invokes pdftk.

An alternative is to use the builtin mapfile if available, or use the positional parameters. The following examples use two functions, print0_files generates the NUL terminated filenames and create_pdf invokes pdftk:

print0_files | create_pdf combinewd2.pdf

The functions are defined as follows

#-- Generate the NUL terminated filenames, numerically sorted
print0_files() {
    find . -maxdepth 1 -type f -print0 |
        sort -k1.3n -z -t '\0'
}

#-- Read NUL terminated filenames using mapfile
create_pdf() {
    mapfile -d ''
    pdftk "${MAPFILE[@]}" cat output "$1"
}

#-- Alternative using positional parameters
create_pdf() {
    local -r pdf=$1
    set --
    while IFS= read -r -d '' f; do set -- "$@" "$f"; done
    pdftk "$@" cat output "$pdf"
}

Discussion

As pointed out in the comments the simple initial answer does not work with filenames containing spaces or other special characters.
The answer by @joeytwiddle does handle special characters, although it does not sort numerically

#-- The following will not sort numerically due to ./ prefix,
#   e.g. filenames 100, 2, 9, 3.14, 10, 1 results in 
#      ./1 ./10 ./100 ./2 ./3.14 ./9
#
find . -maxdepth 1 -type f -print0 |
  sort -zn |
  xargs -0 sh -c 'pdftk "$@" cat output combinewd2.pdf' "$0"

It does not sort numerically due to each filename being prefixed by ./ by the find command. Some versions of the find command support -printf '%P\0' which would not include the ./ prefix. A simpler, portable fix is to add the -d, --dictionary-order option to the sort command so that it considers only blank spaces and alphanumeric characters in comparisons, but might still produce the wrong ordering

#-- The following will not sort numerically due to decimals
#   e.g. filenames 100, 2, 9, 3.14, 10, 1 results in 
#      ./1 ./2 ./9 ./10 ./100 ./3.14
#
find . -maxdepth 1 -type f -print0 |
  sort -dzn |
  xargs -0 sh -c 'pdftk "$@" cat output combinewd2.pdf' "$0"

If filenames contain decimals this could lead to incorrect numeric sorting. The sort command does allow an offset into a field when sorting, sort -k1.3n, one must be careful though in defining the field separator if filenames are to be as general as possible, fortunately sort -t '\0' specifies NUL as the field separator, and the find -print0 option indicates NUL is to be used as the delimiter between filenames, so sort -z -t '\0' specifies NUL as both the record delimiter and field separator– each filename is then a single field record. Given that, we can then offset into the single field and skip the ./ prefix by specifying the 3rd character of the 1st field as the starting position for the numeric sort, sort -k1.3n -z -t '\0'.