using uuencode for email attachments

I have written a program generate & send emails from our data base direct to our customers.
A typically email uses MIME protocols for an HTML body followed by a pdf file attached.

Only some addressees can open the attached pdf.  Many of them get an error when the try to open the pdf.
It looks like success or otherwise depends on the domain that receives the email. For example, [email protected]<mailto:[email protected]> can open the pdf, but [email protected]<mailto:[email protected]> cannot.
The text of the error depends on the platform trying open it, but they have common theme implying some corruption in the attachment
We also have same trouble with spreadsheets.

My program is written in UNIBASIC .  The code

*         writes the HTML body of the email to an ASCII file

*         uses a shell command  uuencode to append a encoded copy of the pdf file (& appropriate MIME messages) to the end of the email message.
There are no libraries to handle this.

We do all this on
Hardware         : IBM Power4
Operating system : RedHat LINUX
O.S. version     : 3.0 Enterprise
UniData version  : 8.2.2

$ uuencode -v
uuencode (GNU sharutils) 4.13.3
Copyright (C) 1994-2013 Free Software Foundation, Inc., all rights reserved.
This is free software. It is licensed for use, modification and
redistribution under the terms of the
GNU General Public License, version 3 or later
    <http://gnu.org/licenses/gpl.html>


please send bug reports to:  [email protected]

Can you advise the best solution?

Chris Allen
Programmer

[cid:[email protected]]
Direct: (02) 8651 5224 | Fax: (02) 9223 3002 | Mobile: 0426 236 771 | Web: www.ncml.com.au<http://www.ncml.com.au/>
Post: PO Box 4475, Sydney NSW 2001
National Credit Management Limited (NCML) ABN 85 008 281 123

This email and any attachments are confidential and may contain information that is legally privileged or protected by copyright. If you are not the intended recipient you may not copy or deliver this message to anyone, and you should destroy this message and kindly notify the sender by reply email. Confidentiality and legal privilege are not waived or lost by reason of mistaken delivery to you. It is the recipient's responsibility to ensure that any attachments are scanned for viruses prior to use.