Missing Characters After Merging PDF Files

Hi,

After merging 10 or more PDF files, I noticed that there were some missing characters in the merged file whenever the words were typed in bold (e.g. letters / symbol "w", "j", "q" and "&") on a consistent basis. I've then checked the individual PDF files but no missing characters were found.

I've never faced this kind of problem in my 2 earlier attempts in March 2016 and April 2016 to merge less than 10 PDF files.  

I am currently using HP ProBook 6460b with Windows 7 SP1.

Any help would be appreciated! Thanks in advance!