Why does my font characters change when I save my work in Acrobat Pro DC ? eg 12345 become /0123

@Karl Heinz Kremer is correct: it's the half-baked way banks, utility companies, and other entities generate massive amounts of PDFs from their systems. Slowly, some banks are correcting the problem.

One trick I use: Use Acrobat's commenting tools, rather than trying to actually edit the PDF. I use the highlighter, sticky note, and comment text to annotate my bank statements. 

A benefit: it keeps the original PDF in tact, in its original form from the bank, so it can be used for legal purposes, such as disputing a charge to your account. If instead you use the edit tools, you've now changed the core of the PDF and it's no longer evidence.

Just a thought.

This very likely has to do subset embedded fonts. Did you combine multiple PDF files into one before trying to edit the file?