Hi mccabe,
I am completely confused by your description of your code. I don't understand how most of what you have described could happen. (Or, since your code isn't working, maybe I'm not confused.)

OK. think I understand the first part of this. You are afraid that some of your directory names contain <newline> characters, so instead of using <newline>s to separate the timestamp and directory name output records from find, you're using a null byte to separate records. You hadn't said anything about <newline>s in filenames (including directories) being a problem, so why add this complexity? It is clearly confusing me and your awk script.

I'm further confused by your choice of variable names. The value assigned to the variable named filename is the pathname of a directory (which I believe is a synonym for folder in your code). The value assigned to the variable named folder is a substring of the value assigned to filename and, if I understand it correctly is also a substring of exactly one PDF file in this directory as well. Is there also supposed to be a directory with a name that exactly matches that substring? Are you trying to copy the PDF file to a regular file with that common substring as its new filename? Are you trying to copy the PDF file into a directory whose name is that common substring?
I don't know what most of that means, but I see absolutely no possibility that this code could do that. The awk code seems to be playing some games with the name of the directory being processed on this time through the loop. The awk code is not given any filenames to process, so it will be reading all of the remaining output from sort as a single partial line of text that contains a NULL byte (meaning it is not text) for each directory that find found. This also means that we will only go through this loop once. The only way that this awk script could ever print anything would be if a line of input in the second input file it processes ends with the string .pdf. But since there is only one partial line of input in the output from sort that awk is reading and there is no second input file for awk to process, that can never happen. Therefore, as we see in the trace output, awk does not print anything and the pdf variable in your script is set to an empty string.

The awk utility is not capable of parsing a PDF file. The awk utility is intended to read lines from text files. PDF files are not text files. The output from sort -z is not a text file either.
Huh? Do you really have directories that have names that end with .pdf?

I'm sorry, but I don't know if you're getting closer. I haven't figured out what directories are involved in what you're trying to do and I haven't figured out what your goals are. I don't even know if you're just trying to move one PDF file or move one PDF file for each directory that is in the directory named /home/cmccabe/folder.
Since you aren't checking the return codes from the functions you call in awk, you don't know whether or not anything matched. You assume that it did and at least sometimes it doesn't.

In post #1 you said you wanted to produce the output:

Code:

Auto_user_S5_0271-95-v5.6_Oncomine_Childhood_Cancer_Research_DNA_and_Fusions.pdf  ---> R_2019_01_30_14_24_53_user_S5-0271-95-v5.6_Oncomine_Childhood_Cancer_Research_DNA_and_Fusions
Auto_user_S5_0271-96-v5.6_Oncomine_Childhood_Cancer_Research_DNA_and_Fusions.pdf ---> R_2019_01_30_14_24_53_user_S5-0271-96-v5.6_Oncomine_Childhood_Cancer_Research_DNA_and_Fusions

but I don't see how that relates to making a copy of the oldest file, why there would be two lines of output related to making a copy of the oldest file, nor why the spacing is different on the desired two lines of output.